U.S. patent number 5,553,491 [Application Number 08/168,093] was granted by the patent office on 1996-09-10 for tire air pressure detecting device.
This patent grant is currently assigned to Nippondenso Co., Ltd.. Invention is credited to Kenji Fujiwara, Masahiko Kamiya, Toshiharu Naito, Takeyasu Taguchi.
United States Patent |
5,553,491 |
Naito , et al. |
September 10, 1996 |
Tire air pressure detecting device
Abstract
It is an object of the present invention to detect a tire air
pressure indirectly with high detection precision. A tire air
pressure detecting device includes a speed sensor for outputting a
signal corresponding to a rotational speed of a tire, and an
electronic control unit which inputs the signal from the speed
sensor and performs predetermined arithmetic operations on that
signal. The electronic control unit calculates a wheel speed based
on the output signal of the speed sensor, performs a frequency
analysis for the calculated vehicle speed, and derives a resonance
frequency corresponding to the unsprung mass in the vertical and
longitudinal directions. The tire air pressure is then detected
based on this resonance frequency.
Inventors: |
Naito; Toshiharu (Okazaki,
JP), Taguchi; Takeyasu (Obu, JP), Kamiya;
Masahiko (Anjo, JP), Fujiwara; Kenji (Kariya,
JP) |
Assignee: |
Nippondenso Co., Ltd. (Kariya,
JP)
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Family
ID: |
27584796 |
Appl.
No.: |
08/168,093 |
Filed: |
December 17, 1993 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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133440 |
Oct 8, 1993 |
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87703 |
Jul 9, 1993 |
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Foreign Application Priority Data
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Nov 11, 1991 [JP] |
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3-294622 |
Feb 4, 1992 [JP] |
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4-18983 |
Feb 10, 1992 [JP] |
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4-57521 |
Mar 16, 1992 [JP] |
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4-55942 |
Apr 17, 1992 [JP] |
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4-125623 |
Apr 17, 1992 [JP] |
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4-125624 |
Apr 20, 1992 [JP] |
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4-128078 |
Apr 20, 1992 [JP] |
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4-128079 |
May 30, 1992 [JP] |
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4-164186 |
Oct 8, 1992 [JP] |
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4-297969 |
Oct 9, 1992 [JP] |
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4-297843 |
Dec 18, 1992 [JP] |
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4-338649 |
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Current U.S.
Class: |
73/146.5 |
Current CPC
Class: |
B60C
23/061 (20130101); B60C 23/062 (20130101) |
Current International
Class: |
B60C
23/06 (20060101); B60C 023/02 () |
Field of
Search: |
;73/146,146.2,146.5,146.8 ;340/445,448 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0083771 |
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Jul 1983 |
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EP |
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2905931 |
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Aug 1980 |
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DE |
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3541494 |
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May 1987 |
|
DE |
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3741818 |
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May 1989 |
|
DE |
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59-85908 |
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Jun 1984 |
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JP |
|
62-87909 |
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Jun 1987 |
|
JP |
|
62-149503 |
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Jul 1987 |
|
JP |
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62-149502 |
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Jul 1987 |
|
JP |
|
63-138229 |
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Jun 1988 |
|
JP |
|
63-305011 |
|
Dec 1988 |
|
JP |
|
2-96613 |
|
Apr 1990 |
|
JP |
|
8803878 |
|
Jun 1988 |
|
WO |
|
9114586 |
|
Oct 1991 |
|
WO |
|
Other References
Patent Abstract of Japan, vol. 11, No. 380, Dec. 11, 1987. .
Patent Abstract of Japan, vol. 10 No. 41, Feb. 18, 1986. .
Patent Abstract of Japan, vol. 12 No. 227, Jun. 28, 1998..
|
Primary Examiner: Chilcot; Richard
Assistant Examiner: Oen; William L.
Attorney, Agent or Firm: Cushman, Darby & Cushman
Parent Case Text
This is a continuation-in-part application of application Ser. No.
08/133,440 filed Oct. 8, 1993, which is a continuation-in-part
application of application Ser. No. 08/387,703 filed Jul. 9, 1993,
now abandoned, which in turn is based on PCT application
PCT/JP92/01457 filed Nov. 10, 1992.
Claims
What is claimed is:
1. A tire air pressure detecting device comprising:
output means installed on a vehicle, for outputting a signal
including a vibration frequency component of a tire while said
vehicle is moving;
deriving means for deriving gains in regard to each frequency of
said vibration frequency component;
extracting means for extracting a plurality of said gains which
correspond to a predetermined frequency band;
estimation means for estimating a resonance frequency at which a
peak gain exists, based on a distribution of said plurality of
gains; and
detecting means for detecting a tire air pressure condition based
on said resonance frequency having said peak gain.
2. A tire air pressure detecting device as set forth in claim 1,
wherein said estimation means includes:
threshold setting means for setting a threshold value from said
peak gain of said plurality of gains, said threshold value being
lower than said peak gain; and
deriving means for deriving two frequencies at which two gains
among said plurality of gains attain said threshold value, and for
designating an intermediate value between said two frequencies as
said resonance frequency.
3. A tire air pressure detecting device as set forth in claim 1,
wherein said estimation means includes:
summing means for summing values of gains within said predetermined
frequency band; and
deriving means for deriving said resonance frequency from a
frequency corresponding to a center of gravity point of said values
of gains summed by said summing means.
4. A tire air pressure detecting device as set forth in claim 1,
wherein said output means comprises a wheel speed sensor for
generating a signal corresponding to a rotation speed of a wheel of
said tire.
5. A tire air pressure detecting device as set forth in claim 1,
wherein said estimation means estimates said resonance frequency
based on vibrations of an unsprung mass of said vehicle, said
vibrations being generated in at least one of a vertical direction
and a longitudinal direction relative to said vehicle.
6. A tire air pressure detecting device as set forth in claim 1,
wherein said detecting means preliminarily stores a resonance
frequency value as a reference value, and detects lowering of a
tire pressure condition from comparing said resonance frequency
estimated by said estimation means with said reference value, where
said reference value corresponds to a normal condition of said tire
air pressure condition.
7. A tire air pressure detecting device comprising:
output means installed on a vehicle, for outputting a signal
including a vibration frequency component of a tire while said
vehicle is moving;
analyzing means for analyzing a distribution of gains in regard to
each frequency of said vibration frequency component;
determining means for determining a frequency at which a peak gain
among said distribution of gains exists, based on said distribution
of gains; and
detecting means for detecting a tire air pressure condition based
on said frequency having said peak gain.
Description
TECHNICAL FIELD
The present invention relates to an air pressure detecting device
which detects an air pressure condition in a vehicular tire.
BACKGROUND ART
Conventionally, as a device for detecting an air pressure in a
vehicular tire, there has been proposed a direct detection device
which uses a pressure responsive member within the tire. However,
because the pressure responsive member must be provided within the
tire, this device results in complicated construction and high
costs.
Therefore, there has been proposed a device for indirectly
detecting the tire air pressure which is based on a detection
signal from a wheel speed sensor. This detection signal represents
a wheel speed for each wheel which is based on a relationship
between the tire radius and the tire air pressure. For instance,
when the tire radius becomes smaller, the tire air pressure is
decreased.
The tire radius may be affected by differences in each tire which
are due to tire wear or traveling conditions such as cornering,
braking, starting or so forth. Furthermore, the radius of many
tires does not change responsively to changes in the tire air
pressure. For instance, when the tire pressure is decreased at a
rate of 1 Kg/cm.sup.2 the corresponding tire radius deformation
magnitude may be only 1 mm. For these reasons, the method for
indirectly detecting the tire air pressure based on the deformation
magnitude in the tire radius, is problematic in that it cannot
always provide accurate tire air pressure detection.
SUMMARY OF THE INVENTION
The present invention is presented to provide accurate tire air
pressure detection in view of the problems set out above.
Accordingly, it is an object of the present invention to provide a
tire pressure detecting device which indirectly detects the tire
air pressure with high detecting precision.
In order to accomplish the above-mentioned object, a tire air
pressure detecting device, according to the present invention,
detects the air pressure of a tire by generating, monitoring, and
adjusting to changes in a signal which contains a vibration
frequency component corresponding to the tire. Variations within a
tire vibration frequency pattern are determined based on that
signal. More specifically, when the tire air pressure varies, the
associated tire spring constant also varies. Consequently, because
the tire vibration frequency component pattern in the signal
containing the tire vibration frequency component is varied through
spring constant variation, the tire air pressure condition can be
determined based on the variation of this pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an illustration showing the orientation of the first
embodiment of the invention;
FIG. 2 is a characteristic chart showing a frequency
characteristics of acceleration for an unsprung mass of a
vehicle;
FIG. 3 is a characteristic chart showing variation of resonance
frequencies of the unsprung mass of the vehicle due to variation of
a tire air pressure in vertical and longitudinal directions;
FIG. 4 is an illustration of the principle of detection of the tire
air pressure in the first embodiment;
FIG. 5 is a chart showing a waveform of an output voltage of a
wheel speed sensor;
FIG. 6 is a chart showing a waveform showing a varying condition of
a wheel speed v which is calculated on the basis of a detection
signal from the wheel speed sensor;
FIG. 7 is a characteristic chart showing a result of frequency
analyzing operation with respect to wheel speed v of the waveform
illustrated in FIG. 6;
FIG. 8 is an illustration of an averaging process of the first
embodiment;
FIG. 9 is a characteristic chart showing a result of frequency
analysis after a moving averaging process in the first
embodiment:
FIG. 10 is a flowchart showing a process of an electronic control
unit of the first embodiment;
FIG. 11 is a characteristic chart showing a relationship between
the tire air pressure and the resonance frequencies in the second
embodiment of the invention;
FIG. 12 is a flowchart showing the difference in process between
the second embodiment and the first embodiment;
FIG. 13 is an illustration showing a construction of the third
embodiment of the invention;
FIG. 14 is a flowchart showing the difference in process between
the third embodiment and the first embodiment;
FIG. 15 is an illustration showing a construction of the fourth
embodiment of the invention;
FIG. 16 is an illustration showing a construction of the fifth
embodiment of the invention;
FIG. 17 is a flowchart showing a process of the electronic control
unit of the sixth embodiment;
FIG. 18 is a timing chart showing a variation of the wheel
speed;
FIG. 19 is a characteristic chart illustrating peaks in gain which
correspond to the integral multiples of wheel rotations per unit
time;
FIG. 20 is an illustration of the control performed by the seventh
embodiment;
FIG. 21 is a flowchart illustrating a principle of the process of
the seventh embodiment;
FIG. 22 is an illustration of the control performed by the eighth
embodiment;
FIG. 23 is a flowchart illustrating a principle of the process of
the eighth embodiment;
FIG. 24 is an illustration of the control performed by the ninth
embodiment;
FIG. 25 is a flowchart illustrating a principle of the process of
the ninth embodiment;
FIGS. 26(a), 26(b) and 26(c) are illustrations of the principle of
the process the tenth embodiment;
FIG. 27 is a flowchart illustrating a principle of the process of
the tenth embodiment;
FIGS. 28(a) and 28(b) are illustrations of the control performed by
the eleventh embodiment;
FIG. 29 is a characteristic chart showing a frequency distribution
of the wheel speed in the eleventh embodiment;
FIG. 30 is a characteristic chart showing predicted gain
distribution of a tire rotation degree component in the eleventh
embodiment;
FIG. 31 is a characteristic chart showing a frequency
characteristics from which the tire rotation degree component is
removed in the eleventh embodiment;
FIG. 32 is a flowchart illustrating a principle of the process of
the eleventh embodiment;
FIGS. 33(a), 33(b) and 33(c) are illustrations of the control
performed by the twelfth embodiment;
FIG. 34 is an illustration for discussion of an outline of the
control performed by the twelfth embodiment;
FIG. 35 is an illustration for discussion of an outline of the
control performed by the twelfth embodiment;
FIG. 36 is a flowchart illustrating the first portion of the
principle of the process of the thirteenth embodiment;
FIG. 37 is a flowchart illustrating the second portion of the
principle of the process of the thirteenth embodiment;
FIG. 38 is a characteristic chart showing a relationship between a
vehicle speed ratio and a gain coefficient;
FIG. 39 is a flowchart illustrating a principle of the process of
the fourteenth embodiment;
FIG. 40 is a characteristic chart showing a relationship between a
vehicle speed and a gain of respective degrees of the frequency
corresponding to the wheel rotation speed per unit time;
FIG. 41 is a flowchart illustrating a principle of the process of
the fifteenth embodiment;
FIG. 42 is a flowchart illustrating a first portion of the
principle of the process of the sixteenth embodiment;
FIG. 43 is a flowchart illustrating a second portion of the
principle of the process of the sixteenth embodiment;
FIG. 44 is a flowchart illustrating a first portion of the
principle of the process of the seventeenth embodiment;
FIG. 45 is a flowchart illustrating a second portion of the
principle of the process of the seventeenth embodiment;
FIG. 46 is a characteristic chart showing a relationship of the
number of data (SMP) in relation to an difference .DELTA.f between
a resonance frequency f.sub.k and a discriminated value f.sub.L
;
FIG. 47 is a characteristic chart showing a relationship of the
number of an averaging process (SUM) with respect to an difference
.DELTA.f between a resonance frequency f.sub.k and a discriminated
value f.sub.L ;
FIG. 48 is a flowchart showing a first portion of the principle of
the process in the eighteenth embodiment;
FIG. 49 is a flowchart showing a second portion of the principle of
the process in the eighteenth embodiment;
FIG. 50 is a flowchart showing a principle of the process in the
nineteenth embodiment;
FIGS. 51(a) and 51(b) are charts showing waveforms of the vehicle
speed in a time sequence calculated by ECU;
FIG. 52 is a characteristic chart showing a relationship between a
wheel speed variation magnitude .DELTA.v and the number of data
(SMP);
FIG. 53 is a characteristic chart showing a relationship between a
wheel speed variation magnitude, .DELTA.v, and the number of the
averaging processes (SUM);
FIG. 54 is a flowchart showing the process of the electronic
control unit of the twentieth embodiment;
FIG. 55 is a timing chart showing a relationship between the wheel
speed and resonance frequency in the twentieth embodiment;
FIG. 56 is a flowchart showing the principle of the process of the
twenty-first embodiment;
FIGS. 57(a) and 57(b) are characteristic charts showing a
relationship between the wheel speed, the tire air pressure and
resonance frequency of the unsprung mass;
FIG. 58 is a characteristic chart showing a relationship between
the tire pressure of the radial tire and stadless tire and
resonance frequency in the unsprung mass;
FIG. 59 is a flowchart showing a process of the ECU of the
twenty-second embodiment;
FIG. 60 is a flowchart showing a process of the ECU of the
twenty-second embodiment;
FIG. 61 is a flowchart showing a process of the ECU of the
twenty-third embodiment;
FIG. 62 is an illustration of the relationship between the tire air
pressure and the resonance frequency and the tire air pressure;
FIG. 63 is an illustration showing an orientation of a tire air
pressure detecting device which includes a set switch;
FIG. 64 is a flowchart of the process of the ECU of the
twenty-fourth embodiment;
FIG. 65 is a graph showing a relationship of an effective rolling
radius and the resonance frequency of the unsprung mass;
FIG. 66 is a first portion of a flowchart of the signal processing
of the electronic control unit in the twenty-fifth embodiment;
FIG. 67 is a second portion of a flowchart of the signal processing
of the electronic control unit in the twenty-fifth embodiment.
FIG. 68 is a graph showing a relationship of the tire air pressure
and the resonance frequency of the unsprung mass;
FIG. 69 is a graph showing a relationship of an effective rolling
radius and the resonance frequency of the unsprung mass;
FIG. 70 is a first portion of a flowchart of the signal processing
of the electronic control unit in the twenty-sixth embodiment;
FIG. 71 is a second portion of a flowchart of the signal processing
of the electronic control unit in the twenty-sixth embodiment;
FIG. 72 is a characteristic chart illustrating fluctuation of the
tire air pressure with respect to the same resonance frequencies
based on the unsprung mass load;
FIG. 73 is a characteristic chart showing a relationship between
resonance frequency difference and the tire air pressure;
FIG. 74 is a characteristic chart showing a relationship between
resonance frequency f.sub.MAX and the resonance frequency
difference f.sub.TH ;
FIG. 75 is a characteristic chart showing a relationship between
resonance frequency f.sub.MAX and the resonance frequency
difference f.sub.TH ; and
FIG. 76 is a flowchart of the signal processing of the electronic
control unit in the twenty-seventh embodiment;
FIG. 77 is a characteristic diagram showing the relationship
between the tire pressure and the unsprung resonance frequency for
the radial tire and the stadless tire;
FIG. 78 is a flowchart of the process performed in the
twenty-eighth embodiment;
FIG. 79 is a characteristic diagram showing the relationship
between the resonance frequency and the wheel speed (gain);
FIGS. 80(a) and 80(b) contain various characteristic diagrams of
the relationship between the resonance frequency and the wheel
speed (gain);
FIG. 81 is a characteristic diagram demonstrating that the
relationship between the resonance frequency and the wheel speed
(gain);
FIG. 82 is a characteristic diagram demonstrating that the
relationship between the tire pressure and the judgment resonance
frequency;
FIG. 83 is a flowchart showing the processes performed in the
twenty-ninth embodiment;
FIGS. 84(a), 84(b) and 84(c) are illustrations of the data
selection process in the twenty-ninth embodiment;
FIG. 85 is an illustration of the gain adjustment in the
twenty-ninth embodiment;
FIG. 86 is a flowchart showing some of the processes performed by
and ECU in the thirtieth embodiment;
FIG. 87 is an illustration of the data adjustment performed in the
thirtieth embodiment;
FIG. 88 is an illustration of the gain adjustment process performed
in the thirtieth embodiment;
FIG. 89 is a flowchart showing, in detail, one example of process
step 170A shown in FIG. 10;
FIG. 90 is a characteristic chart showing a relationship between
the frequency and gain to be used in connection with FIG. 89;
FIG. 91 is a flowchart showing another example of process step 170A
shown in FIG. 10; and
FIG. 92 is a characteristic chart showing a relationship between
the frequency and gain to be used in connection with FIG. 91.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention will be explained hereinafter with reference
to the drawings.
As shown in the overall construction of the first embodiment
illustrated by FIG. 1, wheel speed sensors are provided for each
tire 1a.about.1d of a vehicle. Each wheel speed sensor comprises
gears 2a.about.2d and pick-up coils 3a.about.3d. The gears
2a.about.2d are coaxially mounted on a rotary shaft (not shown) of
each tire 1a.about.1d, and are made from disc-shaped magnetic
bodies. The pick-up coils 3a.about.3d are positioned in close
proximity to the gears 2a.about.2d with a predetermined gap
therebetween for outputting an alternating current (AC) signal
which has a period corresponding to the rotational speed of both
gears 2a.about.2d and tires 1a.about.1d. The AC signal output from
pick-up coils 3a.about.3d is input into a known electronic control
unit (ECU) 4 comprising a wave shaping circuit, ROM, RAM and so
forth. A predetermined signal processing, which includes wave
shaping, is performed. The result is input into display portion 5
which indicates the air pressure condition of each tire,
1a.about.1d, to the driver. The display portion 5 may either
display the air pressure condition of each tire independently, or
it may provide an alarm lamp which is turned on when the air
pressure of a tire is lowered below a reference air pressure.
The tire air pressure detecting device will now be explained
according to the present embodiment.
When a vehicle travels on a paved asphalt road, for example, the
tire is subject to upward and downward (also known as vertical),
forward and backward (also known as longitudinal) forces due to the
fine undulation on the road surface. These cause the tire to
vibrate, where forward and backward references will be referred to
as longitudinal, and upward and downward references will be
referred to as vertical throughout the remainder of the
specification.
FIG. 2 shows a frequency characteristic of the acceleration of the
unsprung mass of a vehicle during tire vibration. As shown in this
figure, the frequency characteristic of the acceleration has peak
values at two points. Point `a` represents the resonance frequency
of the unsprung mass of the vehicle in vertical directions, and the
point `b` represents the resonance frequency of the unsprung mass
of the vehicle in the longitudinal directions.
Since the rubber portion of the tire has a spring constant, the
vertical and longitudinal resonance frequencies are both varied
with variations in tire air pressure. For instance, as shown in
FIG. 3, when the tire air pressure is lowered, the spring constant
of the rubber portion of the tire is also lowered. Consequently,
the resonance frequencies are lowered in both the vertical and
longitudinal directions. Accordingly, by extracting at least one of
the resonance frequencies in those directions from the tire
vibration frequency, the corresponding tire air pressure condition
can be detected.
Therefore, in the present embodiment, the resonance frequencies
corresponding to the vertical and longitudinal directions of the
unsprung mass of the vehicle are extracted from a detection signal
of the wheel speed sensor. This is because, as a result of
extensive study made by the inventors, it has been found that a
detection signal of a wheel speed sensor includes a frequency
component of tire vibration. Namely, as a result of frequency
analysis of the wheel speed sensor detection signal, it has been
determined that two peak values exist as shown in FIG. 4, and that
the two peak values decrease in magnitude when the tire air
pressure is lowered.
In recent years, an increasing number of vehicles have been
equipped with anti-skid control systems (ABS). Since these systems
already have wheel speed sensors for each tire, tire air pressure
within those tires can be detected without any additional sensors.
It should be noted that because most resonance frequency variations
are directly related to tire spring constant variations which are
due to variations in tire air pressure, tire air pressure can be
stably detected without concern for other tire factors, such as
wearing and so forth.
In FIG. 10, there is illustrated a flowchart which shows a process
that is executed by ECU 4. Although ECU 4 performs similar
processes for wheels 1a.about.1d, the flowchart of FIG. 10 shows
the flow of the process with respect to a single wheel. Further, in
the explanation given hereinafter, suffixes for respective
reference numerals are omitted. In the flowchart shown in FIG. 10,
there is illustrated a particular example in which an alarm is
provided for the driver when the air pressure of the tire decreases
to a level below, or equal to, a reference value.
In FIG. 10, at step 100A, wheel speed v is calculated by waving
shaping the AC signal output from pick-up coil 3 (shown in FIG. 5)
to form a pulse interval, and by dividing the pulse interval with
the elapsed period therein. As shown in FIG. 6, wheel speed v
normally contains a large number of high frequency components
including the vibration frequency component of the tire. At step
110A, variation magnitude .DELTA.v of calculated wheel speed v is
compared to reference value v.sub.0. If .DELTA.v is greater than,
or equal to v.sub.0, the result is positive, and the process is
advanced to step 120A. At step 120A, the period .DELTA.T, within
which variation magnitude .DELTA.v of wheel speed v is held in
excess of the reference value v.sub.0, is compared a predetermined
period, t.sub.0. A positive result is obtained when .DELTA.T is
greater than, or equal to, t.sub.0.
The processes of the above-mentioned steps 110A and 120A determine
whether the road surface permits detection of the tire air pressure
according to the detection method of the present embodiment.
Namely, in the present embodiment, detection of the tire air
pressure is performed on the basis of a variation of the resonance
frequency contained in the vibration frequency component of the
tire. Therefore, unless the variation in wheel speed v is
continuously greater than a certain magnitude, sufficient data for
calculation of the above-mentioned resonance frequency cannot be
obtained. It should be noted that in the comparison of step 120A,
period .DELTA.T corresponds to the period during which variation
magnitude .DELTA.v of wheel speed v is equal to, or greater than,
reference value v.sub.0. Also, measurement of period .DELTA.T is
continued when variation magnitude .DELTA.v of wheel speed v is
again equal to or greater than reference value v.sub.0.
If steps 110A and 120A produce positive results, the process is
advanced to step 130A. On the other hand, if either one of steps
110A and 120A are negative, the process returns to step 100A.
At step 130A, a frequency analyzing operation (FFT) is performed
with respect to the calculated wheel speed so that the cycles of
operation N are counted. One example of the result of FFT operation
is shown in FIG. 7.
As shown in FIG. 7, substantially random frequency characteristics
are typically obtained when the FFT operation is performed with
respect to the wheel speed obtained on normal roads. This is a
result of irregularities in the configuration (size and height) of
the small undulations on road surface. Accordingly, the frequency
characteristics may vary for each wheel speed data.
The present embodiment is directed at suppressing variations of the
frequency characteristics by deriving an average value which
corresponds to the results of FFT operation over many operation
cycles. At step 140A, the number of the FFT operation cycles, N, is
compared to the predetermined number, n.sub.0. If the number of
operation cycles does not reach the predetermined number n.sub.0 of
cycles, the processes at steps 100A through 130A are again
executed. On the other hand, when the number of operation cycles
does reach the predetermined number n.sub.0 of cycles, the process
is advanced to step 150A to perform an averaging process.
As shown in FIG. 8, the averaging process is used to derive an
average value of the respective FFT operation results, from which
an average value of gains of respective frequency components are
derived. Therefore, this averaging process is used to reduce the
FFT operation results according to the road surface being
traveled.
However, the above-mentioned averaging process may be problematic
because of noise the maximum peaks exist in the vertical and
longitudinal directions which do not always represent the resonance
frequency gains. To overcome this potential problem, the moving
averaging process set out below is performed at step 160A of the
present embodiment in lieu of the foregoing averaging process.
The moving averaging process of step 160A is performed by deriving
gain Y.sub.n at an nth frequency using the following equation:
Namely, in the moving averaging process, gain Y.sub.n,
corresponding to the nth frequency, is derived as an average value
of the (n+1)th frequency, which is the result of operation in the
preceding cycle, and the (n-1)th frequency which was previously
derived. The result of the FFT operation is a smoothly varying
waveform, and the results of the moving average process are shown
in FIG. 9.
It should be noted that the wave shaping process is not specified
in the foregoing moving averaging process. A low-pass filter can be
employed to obtain the results of the FFT operation.
Alternatively, it is possible to perform a differentiating
operation to determine the wheel speed v before performing the FFT
operation at step 130A. Then, the FFT operation may be subsequently
performed to determine the result of the differentiating
operation.
At step 170A, based on the results of the FFT operation which are
smoothed through the above-mentioned moving average process,
resonance frequency f.sub.K of the unsprung mass in the
longitudinal direction is derived. Then, at step 180A, lowering
difference (f.sub.0 -f.sub.K) is derived from initial frequency
f.sub.0 which is preliminarily set based on the normal tire air
pressure. This lowering difference is compared with a predetermined
difference .DELTA.f, where predetermined difference .DELTA.f
corresponds to an allowable lowest value (e.g. 1.4 kg/m.sup.2) of
the tire air pressure with reference to the initial frequency
f.sub.0. Accordingly, if determination is made at step 180A that
lowering difference (f.sub.0 -f.sub.K) is equal to, or greater
than, predetermined difference .DELTA.f, the tire air pressure is
regarded to be below the allowable lowest value. In this case, the
process is advanced to step 190A and an alarm is displayed for the
driver on display portion 5.
In the above-described embodiment, shown in FIG. 10, a maximum peak
gain may not coincide with resonance frequency f.sub.k which is
derived in step 170A. Therefore, resonance frequency f.sub.k may be
estimated using a technique known as shape estimation
processing.
FIG. 89 illustrates a detailed example of the shape estimation. In
step 171, relative maximum peak gain G.sub.p is derived for the
frequency band within which resonance frequency f.sub.k is expected
to appear. At subsequent step 172, threshold gain G.sub.s is set to
a value which has, for example, 70% the amplitude of peak gain
G.sub.p. Frequencies f.sub.1 and f.sub.2, at which the threshold
value G.sub.s is attained within predetermined frequency band
f.sub.b, are then calculated as shown in FIG. 90.
At step 173, it is determined whether there exist two points (P=2)
within F.sub.b at which the threshold value is attained. Step 174
is performed only when there exists at least two points. In step
174, resonance frequency f.sub.k is set to a frequency which is in
the middle of frequencies f.sub.1 and f.sub.2, where that middle
frequency is designated as f.sub.m. When the threshold value is not
reached at least twice within F.sub.b, step 175 is performed,
wherein a new value for resonance frequency f.sub.k is not derived.
Instead, resonance frequency f.sub.k is estimated based on the
previously derived resonance frequency f.sub.k. Alternatively, in
step 175, resonance frequency f.sub.k may be estimated based on a
plurality of previously derived resonance frequencies (average
value thereof) to account for noise components which are present
when there exist more than two points at which the threshold value
is reached.
Although resonance frequency f.sub.k is estimated through the
setting of just one threshold value G.sub.s in FIG. 89, the
accuracy of such estimation can be further enhanced if ECU 4 has
sufficient memory capacity. That is, in step 172, a plurality of
threshold values (for example, 70% 60% and 50% of G.sub.p) may be
set so that a plurality of frequencies are used which correspond to
respective threshold values attained. Thereafter, at step 174,
intermediate frequencies (f.sub.m1, f.sub.m2 and f.sub.m3) may be
derived for each threshold value in the manner described above and
the average value of such intermediate frequencies may be set to
the resonance frequency f.sub.k. In deriving the average value,
some of the intermediate frequencies which deviate too much may be
eliminated, or the intermediate frequencies may be weighted
respectively in accordance with the threshold values.
Alternatively, as another example, the estimation of resonance
frequency f.sub.k may also be derived using the center of gravity
of a waveform around resonance frequency f.sub.k. That is,
frequency f.sub.c, at which the center of gravity point exists, may
be set equal to resonance frequency f.sub.k (gravity center
processing) in the manner shown by FIG. 91. Specifically, in step
176, sum S of all gains in a predetermined frequency band (hatched
area in FIG. 92) is derived. At subsequent step 177, the gains are
added from the edges of such predetermined frequency band as well
as from frequency f.sub.c at which sum S is equal to S/2. This
frequency represents a point at which the gravity center point
exists and is set as resonance frequency f.sub.k (see FIG. 92).
Preferably, the frequency band used in step 176 should be picked up
to ensure that a peak in the resonance frequency characteristic
resides therein.
It should be noted that although in the foregoing embodiment, an
example is illustrated to detect decreases is the tire air pressure
based on resonance frequency in the longitudinal direction, it is
also possible to detect the tire air pressure based on the
resonance frequency in the vertical direction, or based on
resonance frequencies in both the longitudinal and vertical
directions.
It should also be noted that not all of the ECU processes performed
in the following embodiments differentiate from those in the first
embodiment. Similarly, some aspects of the construction and
orientation of the components used in the following embodiments are
generally common to previously discussed embodiments. Therefore,
only those processes, components and orientations of the following
embodiments which are different from previously discussed
embodiment will be described.
In the second embodiment, abnormal tire air pressures are
determined by directly comparing the air pressures of the tires
with predetermined air pressure value P.sub.o. Therefore, instead
of using a frequency value for comparison, such as the lowering
difference (f.sub.o -f.sub.k) of the first embodiment, this
embodiment uses the derived tire air pressure value.
Accordingly, in the second embodiment, step 180A of the first
embodiment is replaced by steps 182B and 184B of FIG. 12.
In step 182B, a relationship between tire air pressure and
resonance frequency (as shown in FIG. 11) is used to derive the
actual tire air pressure from the detected resonance frequency.
Then, at step 184B, the derived tire air pressure is compared with
allowable minimum value P.sub.o of the preliminarily set tire air
pressure.
When derived air pressure P is below allowable minimum value
P.sub.o, the process is advanced to step 190A where an alarming
process is performed. Otherwise, the process is returned to step
100A where the detection process is repeated.
It should be noted that, in the second embodiment, the tire air
pressure derived in step 182B may be directly displayed on the
display portion.
In the third embodiment, acceleration sensor 11 is positioned on an
unsprung mass member of the vehicle and is used to generate a
signal containing the vibrational frequency component of the
tire.
Accordingly, as shown in FIG. 13, the output generated by wheel
speed sensor of the first embodiment is replaced with that of
acceleration sensor 11 of the embodiment. Correspondingly, the
process shown in FIG. 14 is executed in lieu of step 100A of the
flowchart of FIG. 10. Specifically, step 102 discloses reading an
acceleration signal from acceleration sensor 11 for further
processing.
Because it is possible to derive the resonance frequencies in both
the vertical direction and the longitudinal direction by performing
an FFT operation on the acceleration of the unsprung mass of the
vehicle, an FFT operation may be applied directly to the output
signal produced by acceleration sensor 11.
Consequently, the third embodiment does not require the wave
shaping operations which were needed to condition the output of the
wheel speed sensors of the first embodiment.
In the fourth embodiment of the present invention, vehicle height
sensors are used for detecting a relative displacement between the
body of the vehicle (sprung mass member) and the tire (unsprung
mass member). In this method, vehicle height sensor 20 is used to
generate an output containing the tire vibration frequency
component, thereby replacing the wheel speed sensors of the first
embodiment.
Specifically, the vehicle height sensor, shown in FIG. 15,
generates an output detection signal which is subjected to an
appropriate low-pass filtering process. The resultant signal then
undergoes two differentiating processes to develop a signal which
is representative of a relative acceleration between the vehicle
body and the tire. Finally, this signal is produced as an output
which is operated on in accordance with steps 110A-190A of FIG.
10.
In the fifth embodiment of the present invention, load sensor 30
replaces wheel speed sensor 20 to generate an output signal
containing the tire vibration frequency component. Similar to
height sensor 20 of the fourth embodiment, load sensor 30 detects,
and generates, a signal corresponding to a load between the vehicle
body (sprung mass member) and the tire (unsprung mass member).
As shown in FIG. 16, load sensor 30 is disposed within a piston rod
of a shock absorber, and it comprises a piezoelectric element for
generating a signal whose amplitude corresponds to the applied
load. Load sensor 30 then outputs a signal which corresponds to a
damping force of the shock absorber. The tire air pressure can be
detected by performing signal processing on this output signal, as
performed by the foregoing third embodiment.
As a result of experiments made by the inventors, it has been found
that signals which contain the actual tire vibration frequency
component also contain noise corresponding to unbalances within the
tire, where uneven wearing, standing wave phenomenon and so forth
may cause such unbalances. Furthermore, this noise occurs at
integral multiples of frequency which correspond to both the number
of rotations of the wheel during a unit period of time, and the
resonance frequency of the unsprung mass in the vertical or
longitudinal directions.
Because the resonance frequency of the unsprung mass in the
vertical or longitudinal direction is extracted from the signal
containing the tire vibration frequency component, it is often
erroneously derived in the above-mentioned embodiments. The
accuracy of detection for tire frequency and air pressure, which
are determined using the above-described embodiments, is therefore
undeterminable. For this reason, improvements in the detection
accuracy are sought.
The sixth to fifteenth embodiments are set forth to achieve
improvements for the accurate detection of resonance frequency and
tire air pressure by overcoming and compensating for the problem
set forth above.
In the sixth embodiment, the processes shown in FIG. 17 are
performed, where steps 1000F to 1200F are similar to steps 100A to
120A of the first embodiment.
However, at step 1300F of the sixth embodiment, wheel speed
variation ratio A is derived on the basis of variation magnitude
.DELTA.v.sub.2, where .DELTA.v.sub.2 is based on wheel speed v
within the predetermined period t.sub.02 (t.sub.02
>>.DELTA.T), as shown in FIG. 18.
Then, at step 1400F, wheel speed variation ratio A is compared with
predetermined value A.sub.0.
This comparison is made to determine whether the tire air pressure
is accurately detectable via the method of the present embodiment.
Namely, when variation .DELTA.v.sub.2 is small, the peaks (herein
after referred to as "tire rotation degree components") appear at
integral multiples of a frequency, where the frequency corresponds
to the number of the wheel rotation per unit period, as shown in
FIG. 19.
Therefore, unless wheel speed v varies above a certain magnitude
within the predetermined period, the tire rotation degree component
cannot be removed, and the tire air pressure cannot be accurately
determined.
Accordingly, when wheel speed variation ratio A is determined to be
smaller than predetermined value A.sub.0, the process returns to
step 1000F. However, if it is determined in step 1400F that wheel
speed variation ratio A is equal to, or greater than, predetermined
value A.sub.0, then the process is advanced to steps
1500A.about.1900A where processes similar to those of the first
embodiment are performed.
Then, at step 2000F, resonance frequency f.sub.K is compared with
the upper and lower limit values f.sub.H and f.sub.L of the
resonance frequency of the unsprung mass, where upper limit value
f.sub.H and lower limit value f.sub.L are set corresponding to
allowable upper and lower limit values of the tire air pressure
(e.g. upper limit value is 2.5 kg/cm.sup.2 and the lower limit
value is 1.4 kg/cm.sup.2). When resonance frequency f.sub.K is
determined to be equal to, or greater than, upper limit value
f.sub.H, the tire air pressure is regarded as being in excess of
the allowable upper value. When the resonance frequency f.sub.K is
equal to, or lower than, lower limit value f.sub.L, the tire air
pressure is regarded to be lower than the allowable lower limit
value. In either case, the process is advanced to step 2001F to
perform alarming display to the driver via display portion 5.
Thus, in the sixth embodiment, the FFT operation is used to derive
the tire vibrational frequency component only when wheel speed
variation ratio A is equal to, or greater than, predetermined value
A.sub.0. For this reason, the tire rotation degree component, which
appears while the speed variation ratio A is small, can be
eliminated.
In the seventh embodiment, the processes of steps 1300F and 1400F
of the sixth embodiment are replaced with steps 1310G and 1311G of
FIG. 21.
At step 1310G, variation magnitude .DELTA.v.sub.3 is derived based
on wheel speed v within unit period t.sub.03. At step 1311G,
variation magnitude .DELTA.v.sub.3(H) is compared to each previous
variation magnitude .DELTA.v.sub.3 (1).about..DELTA.v.sub.3 (N-1),
where variation magnitude .DELTA.v.sub.3(H) is derived in the Nth
cycle of step 1310G, and where .DELTA.v.sub.3
(1).about..DELTA.v.sub.3 (N-1) are derived in the 1st.about.(N-1)th
cycles of step 1310G.
If current variation magnitude .DELTA.v.sub.3(H) is equal to any of
the previous variation magnitudes, the process is returned to step
1000F, and no FFT operation is performed.
However, when .DELTA.v(N) is not equal to any of the previous
variation magnitudes, the process is advanced to step 1500A to
perform the FFT operation. It follows that the tire vibration
frequency component, which is subject to the FFT operation, has
unique wheel speed variation magnitudes .DELTA.v.sub.3
corresponding to each cycle.
For example, as shown in FIG. 20, the variation magnitude
calculated in the third cycle, .DELTA.v.sub.3 (3), is equal to
previously calculated variation magnitude from the first cycle,
.DELTA.v.sub.3 (1). Therefore, in the third cycle, .DELTA.v.sub.3
(3) would be eliminated and the process would be returned to 1000F
without further processing. Alternatively, in cycles where the
variation magnitude is not equal to any previously calculated
variation magnitudes, the processing is advanced to step 1500F so
that an FFT operation and subsequent averaging processes may be
performed.
Therefore, the peak appearing in the tire vibration frequency
component maintains the resonance frequency component of the
unsprung mass in longitudinal and vertical directions at the same
frequency. However, tire rotation frequency components appear at
different frequencies, are removed by the FFT operation performed
in step 1500F and subsequent steps.
Thus, although the FFT operation is performed when the current
variation magnitude is different from any of the previous variation
magnitudes, another criteria exists. Namely, the FFT operation is
performed only when average wheel speed v.sub.C(N) is different
from any previous average wheel speed, v.sub.c(1)
.about.v.sub.c(N-1), where V.sub.C(N) is derived in during the Nth
operation and v.sub.c(1) .about.V.sub.c(N-1) are derived prior to
the Nth operation.
In the foregoing sixth and seventh embodiments, the tire rotation
degree component is processed before the FFT operation is
performed.
Alternatively, in the eighth embodiment, the tire rotation degree
component is processed after the FFT operation.
The gain of both the tire vibration frequency component and the
tire rotation degree component are affected greatly by road surface
condition. For instance, as shown in FIG. 22, when the vehicle
travels on a rough or unpaved road, the gain of both the tire
vibration frequency component and the tire rotation degree
component become large.
Therefore, in the eighth embodiment, the FFT operation is used to
derive tire vibration frequency components for each cycle. Then,
the averaging process is performed only on those frequency cycles
which have a maximum gain, v.sub.a, which falls within a
predetermined range V.sub.MAX .about.V.sub.MIN. Because only
consistent FFT operation results are considered by the averaging
process, the influence of the tire rotation degree component after
the averaging process becomes small.
Accordingly, in the eighth embodiment, steps 1300F.about.1600F of
the sixth embodiment are replaced by steps 1320H.about.1323H which
are illustrated in FIG. 23.
At step 1320H, an FFT operation is performed to derive the tire
vibration frequency components. Then, at step 1321H, maximum gain
from among the derived tire vibration frequency components,
v.sub.a, is compared to upper and lower limit values V.sub.MAX and
V.sub.MIN. If V.sub.a is not between these limits, it is not used
for averaging and the process returns to step 1000F. However, if
v.sub.s is between these limits, it will be used for averaging and
the process is advanced to step 1322H where the number of values to
be averaged, N.sub.A, is incremented.
Then, at step 1323H, the number of values to be averaged, N.sub.A,
is compared to predetermined value N.sub.S. If N.sub.A is less than
N.sub.B, the process is returned to step 1000F so that another FFT
operation may be performed. However, when N.sub.A is greater than,
or equal to, predetermined value N.sub.B, the process is advanced
to step 1700F for continued processing.
Thus, data corresponding to travel over rough roads is removed so
that the influence of the tire rotation degree component, which has
large peaks, can be suppressed.
The ninth embodiment features removing excessively large (or small)
tire rotation degree components using a ratio, K.sub.i, between
maximum gain v.sub.a and predetermined gain v.sub.0, where v.sub.a
corresponds to predetermined frequency band f.sub.b.
Namely, in the ninth embodiment, steps 1300F.about.1500F of the
sixth embodiment are replaced with steps 1330I.about.1333I of FIG.
25.
In this embodiment, the FFT operation is performed at step 1330I.
At step 1331I, a coefficient, K.sub.i, is obtained as the ratio of
maximum gain value v.sub.a to predetermined gain v.sub.0, based on
the FFT operation result from step 1330I, where
At step 1332I, the FFT operation results are corrected by
multiplying the tire vibration frequency component derived in the
FFT operation with coefficient K.sub.i. After the FFT operation is
completed, the counter for the number of FFT operations performed
is incremented at step 1333I, and the process is advanced to step
1600F.
As shown in FIG. 24, this process has the effect of normalizing the
gains by v.sub.0 so that no excessively large (or small) data
remains.
The tenth embodiment is directed at removing the tire rotation
degree component directly from the FFT operation result. Similar to
the sixth embodiment, the tenth embodiment utilizes the fact that
the tire rotation degree component is necessarily present within
the frequency range which corresponds to the wheel speed variation
range, or to an integral multiple thereof.
For example, in FIG. 26(a), the wheel speed variation range within
a certain period T.sub.D falls within between minimum value a and
maximum value b. Therefore, deriving frequencies A and B,
corresponding to wheel speeds a and b of FIG. 26(a), requires
estimation of gain between the corresponding frequency values, p
and q. Thus, the portion between p and q, illustrated by the solid
line of FIG. 26(b), is replaced with a straight line, illustrated
by the solid line of FIG. 26(c). Such a series of process is
hereinafter referred to as "interpolation".
To perform this interpolation, steps 1300F.about.1500F of the sixth
embodiment are replaced with steps 1341J.about.1344J of the tenth
embodiment, illustrated in FIG. 27.
At step 1340J, minimum value a and maximum value b are derived,
where they correspond to the wheel speed variation within a certain
period T.sub.D. At step 1341J, frequencies A and B, which
correspond to foregoing minimum value a and maximum value b, are
derived. Then, at step 1342J, the FFT operation is performed.
Because the tire rotation degree components are present within the
frequency range of A.about.B, interpolation is performed by
interpolating between the FFT operation resultant values of
frequencies A and B with a straight line at step 1343J, where the
FFT operation resultant values at A and B are shown as q and p in
FIGS. 26(b) and 26(c). Consequently, the gains corresponding to the
tire rotation degree components of frequency range A.about.B are
reduced.
In step 1344J, the counter for indicating the number of cycles of
the FFT operations performed is incremented, after which the
process is advanced to step 1600F for further processing.
The eleventh embodiment features improved precision for the
interpolation of the above-mentioned tenth embodiment. Namely, in
the tenth embodiment, a distribution of wheel speed frequency (At)
is derived for wheel speeds between maximum wheel speed value b and
minimum wheel speed value a which are varying within period T.sub.D
(FIGS. 28(a) and 28(b)) .
By sorting wheel speeds within wheel speed range of a.about.b, and
determining the number of data points corresponding to each wheel
speed, a distribution of wheel speed frequencies may be derived. As
discussed with respect to the tenth embodiment, the tire rotation
degree component is necessarily present within the frequency range
from A to B. However, it should be noted that the rotational degree
component corresponds to the wheel speed variations in the range
from a.about.b. Therefore, the gain distribution of the tire
rotation degree component is similar to the frequency distribution
of the wheel speed.
In other words, since the tire rotation degree component is
apparent from the number of rotations of the wheel within a unit
period, the wheel speed can be regarded as the number of rotations
of the wheel within that unit period.
Therefore, coefficient K.sub.i (which is the coefficient for
converting the wheel speed frequency A.sub.i into the FFT operated
value v.sub.i at the frequency corresponding to the wheel speed) is
multiplied with wheel speed frequency A.sub.i to predict the
distribution of the gains of the tire rotation degree components
(see FIG. 29 and 30). Subsequently, as shown in FIG. 31, by
subtracting the predicted distribution of gains of the tire
rotation degree components from the result of the FFT operation
within the frequency range of A.about.B, the influence of the tire
rotation degree components are substantially decreased.
Correspondingly, by interpolating between the resultant values q
and p of the FFT operation within the frequency range of A.about.B,
the rotation degree components may also be substantially
decreased.
The foregoing process is illustrated in the flowchart shown by FIG.
32, where steps 1340J.about.1344J of the tenth embodiment are
replaced with steps 1350K.about.1356K of FIG. 32.
At step 1350K, within the period T.sub.D, maximum wheel speed b and
minimum wheel speed a are derived, and the results are stored in
ECU 4. At step 1351K, the stored resultant wheel speeds are
increasingly or decreasingly sorted, and the number of equal speeds
are calculated to determine the distribution of the wheel speed
frequency A.sub.i.
At step 1352K, the frequency corresponding to the wheel speed is
derived. At step 1353K, the gains (.upsilon.i) of the tire rotation
degree components are derived from the distribution of wheel speed
frequency, A.sub.i by multiplying coefficient K.sub.i to
A.sub.i.
Next, at step 1354K, the FFT operation is performed on the gains
(.upsilon.i) of the tire rotation degree components which were
derived in step 1353K. At step 1355K, the gains of the tire
rotation degree component (.upsilon.1) are subtracted from the
resultant value of the FFT operation (v.sub.i) within frequency
range A.about.B, thereby generating a corrected value (v.sub.i ')
corresponding to the FFT operation. The resultant values of the FFT
operation, from which the tire rotation degree components are
removed, are as illustrated in FIG. 31.
At step 1356K, the counter for the number of FFT operation cycles
performed is incremented. Then, the process is advanced to step
1600F for further processing as described in the sixth
embodiment.
The twelfth embodiment features approximating the frequency
distribution of the wheel speed with a convenient configuration,
and subtracting the approximated convenient configuration from the
result of the FFT operation.
It should be noted that, as shown in FIG. 33(a) and 33(b), the
manner used to derive the frequency distribution while the wheel
speed varies from a to b, is the same as that of the eleventh
embodiment.
However, this distribution may be approximated as follows. The most
frequent wheel speed is shown as c. Therefore, the frequency
distribution is approximated by triangle abc' as shown in FIG.
33(c). Then, as shown in FIG. 34 and 35, by multiplying
predetermined coefficient K.sub.i with the triangle abc', predicted
gains (.upsilon.i) of the tire rotation degree components are
derived. Further, by subtracting the derived approximated gains
from the resultant values (v.sub.i) of the FFT operation, the tire
rotation degree components are removed.
It should be noted that no flowchart is provided for this
embodiment because the steps performed are substantially the same
as that of the eleventh embodiment.
On the other hand, in the twelfth embodiment, by using the highest
frequency wheel speed c, an average value of the wheel speed
variation, which is between values a and b may be determined
without removing the tire rotation degree component. Further,
instead of approximating the wheel speed frequency distribution
with triangle abc' statistical distributions, such as normal
distribution, Gaussian distribution and so forth may be
employed.
The thirteenth embodiment is directed toward shortening the
operation process period by determining the gains of the wheel
rotation speed degree components in units of time from a map.
Namely, the referenced map (FIG. 38) relates the initially derived
gain of the degree components of the wheel rotation speed per unit
period (T) to a ratio between vehicle speed V of the subsequent
process, and initially derived vehicle speed V.sub.0.
Explanation of this embodiment will be made with reference to FIGS.
36 and 37.
Steps 101M.about.104M are similar to those in the first embodiment.
At step 105M, vehicle speed V is derived based on the wheel speed
used for the FFT operation process. Note that the vehicle speed,
which is derived immediately after the initiation of process, is
stored in the RAM as vehicle speed V.sub.0. This value is derived
to provide a center speed component of wheel speed v in addition to
the tire vibration frequency component. At step 106M, judgment is
made whether flag F is set to "1".
It should be noted that flag F is reset to "0" only in response to
the turning OFF of an ignition switch. Therefore, in the first
process after turning ON the ignition switch, this flag is set to
"0" so that a negative judgment is made in step 106M to advance the
process to step 107M.
At step 107M, vehicle speed V.sub.0 is subject to a frequency
conversion to obtain a primary frequency which corresponds to the
number of rotations of the wheel within a unit period.
Additionally, integral multiples of the primary frequency are
computed.
At step 108M, gains JV.sub.1 .about.JV.sub.i of the tire rotation
degree components are read into the RAM based on the FFT operation
results. At step 109M, flag F is set to "1" and the process is
returned to step 101M.
Note that flag F is set to "1" so that the processes performed in
steps 107M and 108M are executed only once, immediately after
starting. Accordingly, the process is directly advanced to step
110M in each subsequent cycle, where a vehicle speed rate
(V/V.sub.0) which is relative to the vehicle speed V.sub.0 is
derived. At step 111M, gain coefficients K.sub.1 .about.K.sub.i are
derived by reading a gain coefficient which corresponds to the
vehicle speed rate (V/V.sub.0), from a map shown in FIG. 38, where
the map is preliminarily stored in ECU 4.
At step 112M, gains dV.sub.1 .about.dV.sub.i are derived on the
basis of both the determined gain coefficients K.sub.1
.about.K.sub.i, and the gains of the tire rotation degree
components JV.sub.1 .about.JV.sub.i from step 108M. At step 113M,
gains dV.sub.1 .about.dV.sub.i are subtracted from the results of
the FFT operation to eliminate the influence of the tire rotation
degree components. The processes following step 113M are similar to
those in the foregoing embodiments.
In the fourteenth embodiment a predetermined relationship between
gain dV and vehicle speed V.sub.x is used to derive the gain at
various wheel speeds. The derived gain corresponding to each
integral multiple of the primary frequency degree is then
subtracted from the results of the FFT calculation so that the
influence of the tire rotation degree component may be
eliminated.
The foregoing process is illustrated in flowchart shown by FIG. 39,
where steps 105M.about.114M of the thirteenth embodiment are
replaced with steps 205N.about.209N.
At step 205N, vehicle speed V.sub.X is derived from the results of
the FFT operation performed in step 104M of the thirteenth
embodiment. At step 206N, a frequency conversion is performed on
derived vehicle speed V.sub.X to determined the primary frequency
of the tire rotation degree component, and to obtain frequencies
which correspond to the integral multiples of the primary
frequency. At step 207N, the gains which correspond to respective
degrees dV.sub.1 .about.dV.sub.i of vehicle speed V.sub.X, are
derived from a map (FIG. 40) which is preliminarily stored in ECU
4. At step 208M, gains dV.sub.1 to dV.sub.i of the respective
integral multiple frequencies are subtracted from the results of
the FFT operation, thereby eliminating the influence of the tire
rotation degree component.
In the fifteenth embodiment, the frequency of the tire rotation
degree component is directly removed using a plurality of band-pass
filters. This is explained with reference to the flowchart shown in
FIG. 41.
In steps 301o to 303o, vehicle speed V.sub.X is both derived from
the wheel speed and frequency converted. Thus, the frequency range
of the tire rotation degree component is obtained. At step 304o, a
band frequency (f.sub.a .about.f.sub.b) of band-pass filter
(B.P.F.) F.sub.1 is used to set the band frequencies of a plurality
of band-pass filters, F.sub.2 .about.F.sub.i. Specifically, the
band frequency of band-pass filters F.sub.2 .about.F.sub.i are
respectively set as integral multiples of the band-pass frequency
range, f.sub.a .about.f.sub.b, of band-pass filter F.sub.1.
Then at step 305o, the original waveform is passed through the
respective band-pass filters F.sub.1 .about.F.sub.i to obtain a
time-based waveform, where the resultant waveform is not influenced
by the tire rotation degree components. Using this waveform, the
FFT operational process and subsequent averaging processes, which
are explained with respect to the thirteenth and the fourteenth
embodiments, are performed to derive resonance frequency f.sub.K.
Then, judgments can be made concerning the tire air pressure.
3sixteenth
The tire rotation degree component may also be removed by
performing an FFT analysis on the filtered waveform and subtracting
the results from the original waveform.
However, by using frequency analysis (FFT operation) to extract the
resonance frequency, many summing and multiplying operations must
be performed, resulting in prolonged operation.
Therefore, the sixteenth to nineteenth embodiments are provided to
modify the FFT operation periods depending upon both the required
response characteristics, and the required detection accuracy of
the tire air pressure.
Foregoing FFT operations may have been performed by reading a
predetermined amount of data into the RAM of ECU 4, and repeating
summing and multiplying operations on all of this data so that the
resonance frequency can be extracted. However, because the
resonance frequency is known in the present invention, a frequency
range, w.sub.f, within which the FFT operation is performed can be
preliminarily set. Accordingly, if a large amount of data is read
into RAM of ECU 4, the frequency range many be divided into many
(n.sub.f) smaller frequency ranges so that the frequency resolution
(w.sub.f /n.sub.f) and detecting precision are improved for each
frequency range.
However, reading a large number of data into RAM, as set forth
above, requires a longer period for obtaining one result of the FFT
operation (hereinafter referred to as "FFT data"), and leads to a
heavier load on ECU 4.
Additionally, in order to achieve a high frequency resolution, a
large number of FFT data must be provided for the averaging
process.
Alternatively, when the required frequency resolution is lower, the
amount of FFT data required for such averaging processes is
reduced.
The sixteenth to nineteenth embodiments utilize the foregoing
properties of the FFT operation.
For instance, when less detection accuracy is required, the number
of averaging processes are reduced while the difference between the
derived resonance frequency and the reference value is large.
Therefore, quicker response to relatively swift variation of the
tire air pressure may be obtained by shortening the operation
period of the FFT data.
On the other hand, when the tire air pressure is close to the
reference value, the number of FFT data to be read into RAM is
increased, thereby increasing the number of the averaging process,
the frequency resolution, and the detection accuracy.
The processes of the sixteenth embodiment are illustrated by FIG.
42, where two different levels of specification values are
determined for use in the FFT operation. Accordingly, when the tire
air pressure approaches reference value f.sub.L, the specification
values for the FFT operation are changed. The signal extraction
period is expanded, and both the amount of data to be sampled (SMP)
and the number of the averaging process cycles (SUM) are increased.
Consequently, the frequency resolution and tire air pressure
detecting precision are also increased, thereby decreasing the
possibility of erroneous detection.
On the other hand, when the tire air pressure does not approach
reference value f.sub.L, the tire air pressure detecting process is
performed in a shorter period under the specification of the FFT
operation for lower frequency resolution and higher response
characteristics.
At step 101P of FIG. 42, specification values for the FFT operation
are read, where the specification values for the operation are
initially set for lower detection accuracy.
At subsequent steps 102P.about.105P, processes similar to those
described in steps 100A.about.130A of the first embodiment are
performed.
At step 106P, the number of FFT operations performed (N.sub.S) is
compared to a predetermined number (SNM). If the number of
operation cycles has not reached the predetermined number, then
steps 102P.about.105P are repeated. On the other hand, when the
number of the operation cycles has reached the predetermined
number, the process is advanced to perform the averaging process at
step 107P, moving averaging process at step 108P, and derivation of
the resonance frequency f.sub.K at step 109P.
Then, as shown at step 110P of FIG. 43, a difference between
derived resonance frequency f.sub.K and predetermined air pressure
lowering reference value f.sub.L is derived, where the difference
is referred to as .DELTA.f. At step 111P, the difference is
compared to preset value f.sub.w to determine if the tire air
pressure has decreased to a level near reference value f.sub.L so
that an increase in detection accuracy is required.
When it is determined in step 111P that the difference is less than
f.sub.w, the number of sample data values (SMP), and the number of
averaging process cycles (SUM), are increased to achieve higher
detection accuracy. At step 112P, the status of flag F is checked,
where this flag is set to zero only once, namely the first time it
is checked after initiation of process.
When the flag is set to zero, the process is advanced to step 113P
to update the number of sample data to m.sub.L and to update the
number of averaging process cycles to N.sub.L, where m.sub.L is
less than m.sub.S, and where N.sub.L is greater than N.sub.S. Then,
at step 114P, flag F is set to "1" and the process is advanced to
step 115P.
Otherwise, when either resonance frequency f.sub.k is determined to
not be close to reference value f.sub.L in step 111P, or when the
status of flag F is determined to be equal to "1" in step 112P, no
increase in the detection accuracy is required. Accordingly, the
process advances directly to step 115P.
At step 115P, the most recent resonance frequency calculated,
f.sub.K, is compared to reference value f.sub.L. If resonance
frequency f.sub.K is determined to be lower than, or equal to,
reference value f.sub.L, the process is advance to step 116P to
generate an alarm indicating decreased tire air pressure.
Otherwise, the process is returned to step 102P to repeat the
above-mentioned process.
After starting the process, if resonance frequency corresponding to
the tire air pressure gradually approaches the reference value
f.sub.L, and the specification values of the FFT operation are
updated, air pressure will not be supplied to the tire until the
vehicle stops.
Therefore, steps 112P and 114P are provided in order to avoid
redundant processing of step 113P. Specifically, flag F is set to
"1" in step 114P to indicate that the specification values of the
FFT operation have been updated. Thus, when it is detected in step
112P that the specification values have been previously updated,
these steps are not performed and the air pressure lowering
judgment process of step 115P is immediately performed.
The seventeenth embodiment is directed at expanding the signal
extraction period when .DELTA.f becomes smaller than f.sub.w2 so
that both the SMP and the SUM are increased, where .DELTA.f is the
difference between resonance frequency f.sub.K and reference value
f.sub.L, and f.sub.W is a predetermined value. In this embodiment,
the values of SMP and SUM which correspond to .DELTA.f are
determined by using predetermined relationships, such as those
shown in FIGS. 46 and 47.
In other words, the difference .DELTA.f of resonance frequency
f.sub.K is related to both the number of data (SMP) shown in FIG.
45, and to the number of data (SUM) shown in FIG. 46, where each
map is preliminarily stored in ECU 4. Therefore, values
corresponding to SMP and SUM are determined on the basis of the FFT
operation result.
Consequently, the number of levels for specification values of the
FFT operation are increased, and further improvements in tire air
pressure detecting precision are achieved.
The foregoing process is illustrated in the flowchart shown by
FIGS. 44 and 45. In this flowchart, steps 112P.about.114P of the
sixteenth embodiment are replaced by step 211Q of the present
embodiment.
Accordingly, at step 211Q, the specification of the FFT operation
is updated with values of SMP and SUM based on .DELTA.f which was
derived in step 210Q, where SMP is obtained from the map of FIG. 46
and where SUM is obtained from the map of FIG. 47. These values are
then used to control the FFT operation as described with respect to
the sixteenth embodiment.
It should be noted that, similar to step 101P of the sixteenth
embodiment, step 201Q reads out the FFT operation specification
values which result in the lowest detecting precision. For
instance, the lowest value for SMP is shown as DAT4 in FIG. 46, and
the lowest value for SUM is shown as N.sub.i in FIG. 47.
The eighteenth embodiment is directed at decreasing the response
time for air pressure detection by decreasing FFT operation time.
Thus, the alarm may be quickly engaged when the tire air pressure
is decreased in the acceleration state. This becomes particularly
important when tire air pressure changes rapidly, such as when
entering highways and the like.
The eighteenth embodiment will be explained with reference to the
flowcharts of FIGS. 48 and 49.
At steps 301R and 302R, the initial specification values for the
FFT operation are read out and wheel speed v is calculated. At step
303R, the FFT operation and integration of the number of operation
cycles are performed.
Then, at step 304R, vehicle speed V and predetermined value V.sub.H
are compared. If V is greater than, or equal to, V.sub.H, the
process is advanced to step 305R, where the status of flag F is
checked. Because flag F is adapted to be reset only in response to
turning OFF of the ignition, the process is advanced to step 306R
only after the first status check.
At step 306R, the period (T) in which vehicle speed V reaches set
value V.sub.H, is compared to a calculated period for performing an
FFT operation using the initial specification values. Specifically,
the period for performing an FFT operation using the initial
specification values is calculated as t.times.m.sub.S
.times.N.sub.S, where t is a sampling period, m.sub.S is number of
data and N.sub.S is the number of FFT data.
The process is advanced to step 307R when period T is less than, or
equal to, the operational period, because this condition indicates
that the period taken to reach set value V.sub.H was shorter than
the FFT operation period.
Normally, such cases frequently appear during an acceleration state
before entry into high speed traveling. Accordingly, if the tire
air pressure is low, the FFT operation must be quickly processed to
generate a timely alarm indicating the state of low tire air
pressure.
Therefore, at step 307R, a possible number N.sub.S ' (truncated at
radix point) is derived which corresponds to the maximum number of
FFT operation cycles per period T. Then, at step 308R, the number
of the FFT data (SUM) is set equal to N.sub.S '.
Flag F is set to "1" at step 309R, the averaging process is
performed at step 310R, the moving averaging process is performed
at step 311R, and resonance frequency f.sub.K is calculated at step
312R, where resonance frequency f.sub.K is performed based on the
foregoing number N.sub.S ' of the averaging process cycles.
Steps 313R and 314R are then performed to determine if the tire air
pressure has been decreased below lower limit f.sub.L and to
display an alarm if it has.
Note that when V is less than V.sub.H at step 304R, period T is
greater than the calculated operational period at step 306R, or
when the flag is set equal to "1" in step 305R then the number of
operation cycles is compared to predetermined specification value
SUM at step 315R. If the number of operation cycles are greater
than, or equal to SUM, then the process is advanced to the
averaging process of step 310R.
However, if either f.sub.K is greater than f.sub.L at step 313R, or
the number of operation cycles is less than SUM, then the tire air
pressure detecting processes after step 301R are again
performed.
It should also be noted that while the present embodiment varies
the specification values of the FFT operation based on the vehicle
speed V, it is possible to vary the specification according to a
vehicle speed variation rate.
The nineteenth embodiment will be explained with reference to the
flowchart of FIG. 50 and waveforms of FIGS. 51, 52 and 53. FIGS.
51(a) and 51(b) show waveforms in a time sequence of vehicle speed
v which is calculated by ECU 4. It should be noted that in the
time-based waveform, a low frequency signal component of the wheel
speed signal is isolated by a filter. As shown in FIGS. 51(a) and
(b), on a relatively smooth road, the variation magnitude .DELTA.v
of the wheel speed is small, and on a rough road, the variation
magnitude becomes large.
Resonance frequency f.sub.K is adapted to detect the resonation
phenomenon of the unsprung mass. It should be noted that on rough
roads, resonation appears with a large magnitude, thereby
permitting easy detection of resonance frequency f.sub.K using
various specification values. Accordingly, this embodiment allows
for quick detection of decreases in tire air pressure while
traveling on non-paved roads, or during off-road traveling.
Conversely, on smooth roads, resonation appears with a small
magnitude, thereby requiring increased specification values for
higher detection precision.
FIG. 52 shows a map of SMP relative to variation magnitude .DELTA.v
of the wheel speed, and FIG. 53 shows a map of SUM relative to the
variation magnitude .DELTA.v of the wheel speed. Both maps are
stored in ECU 4.
Wheel speed v is calculated and variation magnitude .DELTA.v of the
wheel speed is derived in steps 402S and 403S, respectively. Using
preliminarily set wheel speed variation magnitudes .DELTA.v.sub.1
and .DELTA.v.sub.2, the road surface is evaluated as rough or
smooth.
Then, at step 404S, the specification of the FFT operation is
updated with values of SMP and SUM based on the value for .DELTA.v
derived in step 403S, where SMP is obtained from the map of FIG. 52
and SUM is obtained from the map of FIG. 53. These values are then
used to control the FFT operation as described with respect to the
sixteenth embodiment.
It should be noted that multiple wheel speed variation magnitudes
.DELTA.v.sub.1 and .DELTA.v.sub.2, which are used for
discrimination between the rough road and the smooth road, are
simultaneously set depending upon the road surface condition.
The twentieth embodiment is directed at compensating for
temperature changes within the tire.
The process of this embodiment is shown in FIG. 54, where steps
100T.about.170T are similar to those of the foregoing embodiments.
However, when the process at step 170T is executed, initially
calculated resonance frequency f.sub.K is stored as an initial
resonance frequency, f.sub.S '.
When a tire is heated, the air in the tire is expanded, and the air
pressure within the tire increases. Because heating results in
changes in air pressure during a state of constant air volume, it
is not possible to accurately detect tire air pressure based solely
on the actual amount of the air in the tire.
Therefore, through steps 180T.about.240T, unsprung mass resonance
frequency upper limit value f.sub.H and unsprung mass resonance
frequency lower limit value f.sub.L are corrected so that accurate
detection of the tire air pressure may be performed irrespective of
tire heating.
At step 180T, wheel speed v is compared to predetermined wheel
speed v.sub.T, and rising difference .DELTA.f (f.sub.K -f.sub.S) is
compared to predetermined difference .DELTA.f.sub.0. It should be
noted that predetermined difference .DELTA.f.sub.0 is preliminarily
set based on initial resonance frequency f.sub.S to account for
heating characteristics of the tire.
When wheel speed v exceeds predetermined speed v.sub.T, and rising
difference .DELTA.f is equal to, or greater than, predetermined
difference .DELTA.f.sub.0, the vehicle is determined to be running
at high speed and the resonance frequency is increased.
Accordingly, the tire can be regarded as heated. For this reason,
the process is advanced to step 190T where flag F is set equal to
"1", thereby indicating that f.sub.H and f.sub.L are in a
correction state. Next, the process is advanced to step 200T to
correct f.sub.L and f.sub.H due to the temperature change. Namely,
f.sub.L and f.sub.H are corrected through the addition of rising
difference .DELTA.f to each.
Otherwise, the process is advanced to step 180T to step 210T where
wheel speed v is compared to predetermined speed v.sub.T, and
rising difference .DELTA.f is compared to predetermined difference
f.sub.0. If wheel speed v is equal to, or lower than, predetermined
speed v.sub.T, while rising difference .DELTA.f is smaller than
predetermined difference .DELTA.f.sub.0, the vehicle is determined
to be running at a low speed resulting in decreased resonance
frequency. Accordingly, the tire can be regarded as not heating so
that the process is advanced to step 230T where flag F is reset to
"0" thereby indicating that the correction state is no longer
active. At step 240T, unsprung mass resonance frequency upper limit
value f.sub.H is set equal to unsprung mass resonance frequency
upper limit value f.sub.H ', and unsprung mass resonance frequency
lower limit value f.sub.L is set equal to unsprung mass resonance
frequency lower limit value f.sub.L '.
However, if, in step 210T, wheel speed v is higher than
predetermined speed v.sub.T, or if rising difference .DELTA.f is
larger than predetermined difference f.sub.0, the temperature of
the tire becomes unclear. Therefore, in the present embodiment, the
preceding values for f.sub.L and f.sub.H are maintained without
correction.
Then, the process is advanced to step 220T to check the status of
flag F, thereby determining the correction state. If in correction
state (F=1), the process is advanced to step 200T to continue
correction. On the other hand, if not in correction state (F=0),
the process is advanced to step 240T where no correction is
performed.
The following examples will demonstrate why the above correction is
dependent upon the correction state flag. When the vehicle speed V
exceeds predetermined speed V.sub.T, rising difference .DELTA.f is
smaller than predetermined difference f.sub.0, and the correction
state is enabled (F=1), it can be determined that rising difference
.DELTA.f is temporarily lowered. However, when not in correction
(F=0), the same situation is regarded as being caused by an
increase in wheel speed due to temporary acceleration of the
vehicle.
A timing chart of the processes of steps 180T.about.240T, set forth
above is shown in FIG. 55. As illustrated in this timing chart,
when wheel speed v becomes higher than predetermined speed v.sub.T,
and rising difference .DELTA.f becomes greater than predetermined
difference .DELTA.f.sub.0, correction is initiated. However, once
initiated, correction is not released until wheel speed v becomes
lower than predetermined speed v.sub.T, and rising difference
.DELTA.f becomes smaller than predetermined difference
.DELTA.f.sub.0.
Additionally, note that rising difference .DELTA.f can be an
initially set value instead of the derived value (f.sub.K
-f.sub.S).
Furthermore, although the foregoing embodiment performs correction
of both unsprung mass resonance frequency upper limit value f.sub.H
and unsprung mass resonance frequency lower limit value f.sub.L for
each wheel independently, it may be possible to perform correction
for both values simultaneously on all wheels while rising
difference .DELTA.f exceeds the predetermined difference
.DELTA.f.sub.0.
For instance, similar to the process of step 200T, rising
difference .DELTA.f may be added to both f.sub.H and f.sub.L which
correspond to the wheel in which rising difference .DELTA.f exceeds
predetermined difference .DELTA.f.sub.0. Alternatively, for the
wheels in which rising difference .DELTA.f does not exceed
predetermined difference .DELTA.f.sub.0, the correction may be
performed with an average value, .DELTA.f.sub.ave, of the rising
differences of the wheel .DELTA.f.
Until the predetermined vehicle speed is reached, the initial
resonance frequency f.sub.s may be determined with either an
average value of resonance frequencies derived, or, with the final
value of resonance frequencies derived, instead of setting it equal
to f.sub.K.
It should be noted that while the above-mentioned embodiment shows
an example of detecting decreases in tire air pressure based only
on the resonance frequency of the unsprung mass of the vehicle in
the longitudinal direction, it is possible to detect decreases in
the tire air pressure based only on the resonance frequency in the
vertical direction, or, in the alternative based on both of the
resonance frequencies in the vertical and longitudinal
directions.
Similar to the twentieth embodiment, the twenty-first embodiment is
directed at correcting unsprung mass resonance frequency upper
limit value f.sub.H and unsprung mass resonance frequency lower
limit value f.sub.L. However, in addition to the effects gained in
the twentieth embodiment, the twenty-first embodiment is directed
at preventing standing wave phenomenon or bursting which are caused
by increased vehicle speeds.
Typically, a tire is rated for a range of vehicle speeds depending
upon its grade. Thus, the minimum air pressure (P.sub.0) and
maximum air pressure (P.sub.Z) are based on the vehicle speed and
are stored as reference values.
However, when the vehicle speed is increased while the tire air
pressure is low, bursting or standing wave phenomenon may occur. To
prevent this, the overall allowable tire pressure range is
increased by raising both P.sub.0 and P.sub.A.
Accordingly, the twenty-first embodiment is directed at modifying
initial unsprung mass resonance frequency upper limit value f.sub.H
" and initial unsprung mass resonance frequency lower limit value
f.sub.L " based on the vehicle speed.
Accordingly, in the twenty-first embodiment, steps 171U.about.177U
of FIG. 56 are added between steps 170T and 180T of the twentieth
embodiment.
At step 171U, wheel speed v is compared with first speed
v.sub.G.
Wheel speed v is not considered excessive when it does not exceed
first speed v.sub.G, and correction is unnecessary. When this is
the case, the process is advanced to step 172U, where the initial
unsprung mass resonance frequency upper limit value f.sub.H " is
set equal to unsprung mass resonance frequency upper limit value
f.sub.H ', and unsprung mass resonance frequency lower limit value
f.sub.L " is set equal to unsprung mass resonance frequency lower
limit value f.sub.L '.
However, if wheel speed v exceeds the first speed v.sub.G in step
171U, the process is advanced to step 173U to compare wheel speed v
to a second value, v.sub.H.
If wheel speed v does not exceed second value v.sub.H, the process
is advanced to step 174U to perform correction. Namely, initial
unsprung mass resonance frequency upper limit value f.sub.H " is
corrected through the addition of .DELTA.Q' to obtain unsprung mass
resonance frequency upper limit value f.sub.H ', and initial
unsprung mass resonance frequency lower limit value f.sub.L " is
corrected through the addition of correction value .DELTA.Q to
obtain unsprung mass resonance frequency lower limit value f.sub.L
'.
However, if wheel speed v exceeds second speed v.sub.H in step
173U, the process is advanced to step 175U to compare wheel speed v
to a third value, v.sub.V.
If wheel speed v does not exceed third speed v.sub.V at step 175U,
unsprung mass resonance frequency upper limit value f.sub.H ' and
unsprung mass resonance frequency lower limit value f.sub.L ' are
corrected before the heating dependent correction by adding
correction value .DELTA.H' to unsprung mass resonance frequency
upper limit value f.sub.H " and by adding the correction value AH
to unsprung mass resonance frequency lower limit value f.sub.L
".
However, if the vehicle speed exceeds the third speed v.sub.V, the
process is advanced to step 177U to derive unsprung mass resonance
frequency upper limit value f.sub.H ' and unsprung mass resonance
frequency lower limit value f.sub.L ' through the addition of
correction value .DELTA.v' to the unsprung mass resonance frequency
upper limit value f.sub.H ' and correction value .DELTA.v to the
unsprung mass resonance frequency lower limit value f.sub.L ".
The result of the foregoing processes of steps 171U.about.177U may
be illustrated as shown in FIGS. 57(a) and 57(b). When wheel speed
v is lower than predetermined speed v.sub.G, initial unsprung mass
resonance frequency upper limit value f.sub.H " is set equal to
unsprung mass resonance frequency upper limit value f.sub.H ', and
initial unsprung mass resonance frequency lower limit value f.sub.L
" is set equal to unsprung mass resonance frequency lower limit
value f.sub.L '. However, when wheel speed v is increased, f.sub.H
" and f.sub.L " are each corrected so that f.sub.H ' and f.sub.L '
are gradually increased.
Correspondingly, as shown in FIG. 57(a), allowable lower limit
value P.sub.0 and allowable upper limit value P.sub.Z are also
increased to raise the overall allowable range of the tire air
pressure, thereby preventing the bursting or the standing wave
phenomenon.
The above-mentioned embodiments are established in view of any
single type of tire. However, if any of the tires are different in
type, the rating for tire air pressure may also be different.
Therefore, the reference value (unsprung mass resonance frequency)
for determining decreases in tire air pressure may fluctuate
corresponding to tire type.
For this reason, the reference value for discriminating
abnormalities in the tire air pressure must be determined depending
upon the type of tire used. Results of a study made by the
inventors indicate that there are definite differences in the tire
air pressure, also known as unsprung mass resonance frequency
characteristics, between a normal radial tire and a stadless tire
(winter tire), as shown in FIG. 58.
Specifically, the fluctuation range of the unsprung mass resonance
frequency of the normal radial tire (hereinafter simply referred to
as radial tire) is designated by reference A. This range is higher
than the fluctuation range of the unsprung mass resonance frequency
of the stadless tire, designated by reference B. Note that this
difference may depend either on the tire manufacture (brand), or on
the weight of the wheel to which the tire is equipped.
A.sub.max and B.sub.max show the upper limit characteristics of the
fluctuation in the case where the lightest wheel is employed, and
A.sub.min and B.sub.min show the lower limit characteristics of the
fluctuation in the case where the heaviest wheel is employed. The
difference between the maximum and minimum values is a result of
the proportional relationship of unsprung mass resonance frequency
f to .cent.k/m (where m is an unsprung mass weight, k is a spring
constant of the tire).
Assuming that the air pressure limits for (kg/cm.sup.2) alarm due
to change in tire air pressure are defined by lower limit P.sub.L
and upper limit P.sub.H, then the reference resonance frequency
(unsprung mass resonance frequency) f.sub.L for determining the
radial tire air pressure becomes f.sub.RA. Similarly, the reference
resonance frequency f.sub.L of the stadless tire becomes f.sub.ST.
In this case, the minimum air pressure, as defined in JIS standard
(1.4 kg/cm.sup.2) can be used for P.sub.L, and the maximum air
pressure, as defined in JIS standard (2.5 kg/cm.sup.2) can be used
for P.sub.H.
In the twenty-second embodiment, using various combinations of two
switches, 6a and 6b, the type of the tires equipped on the two
front wheels and two rear wheels can be determined. Then, reference
resonance frequencies can be correspondingly set. Therefore, even
when the type of the tires are changed, the air pressure condition
of the tires can be accurately detected.
An example of the processes performed in the twenty-second
embodiment are shown in the flowcharts illustrated by FIGS. 59 and
60.
At step 101V, the status of flag F is checked, where it is reset to
"0" only when the ignition switch has been turned OFF. Accordingly,
the result of step 101V is negative only immediately after signal
processing begins so that the process proceeds to step 102V.
Otherwise, the processes of this embodiment are foregone.
At step 102V, determination is made whether both of selection
switches, 6a and 6b, are in ON state. If both are in ON state,
judgment is made in step 105V that stadless tires are used on all
four wheels. Therefore, reference resonance frequency f.sub.L is
set equal to f.sub.ST for all four wheels at step 105aV.
If both selection switches are not in the ON state at step 102V,
the process is advanced to step 103V to determine whether both
selection switches 6a and 6b are in the OFF state. If both of the
switches are OFF, judgment is made in step 106V that radial tires
are used on all four wheels. Therefore, reference resonance
frequency f.sub.L is set equal to f.sub.RA for all four wheels at
step 106aV.
If both selection switches are not in the OFF state, the process is
advanced to step 104V. If in step 104V, selection switch 6a is
determined to be in the OFF state, selection switch 6b must
necessarily be ON from the results of prior tests. If this is the
case, the process advances to step 107V, where judgment is made
that radial tires are equipped on the two front wheels, and
stadless tires are equipped on the two rear wheels. Therefore,
reference resonance frequency f.sub.L for the two front wheels is
set equal to f.sub.RA, and reference resonance frequency f.sub.L
for the two rear wheels is set equal to f.sub.ST in step 107aV.
If in step 104V, selection switch 6a is determined to be in the ON
state, judgment is made at step 108V that stadless tires are
equipped on the two front wheels, and radial tires are equipped on
two rear wheels. Therefore, at step 108aV, resonance frequency
f.sub.L for the two front wheels is set equal to f.sub.ST, and
reference resonance frequency f.sub.L for the two rear wheels is
set equal to f.sub.RA.
Consequently, only one portion of the processes described by steps
105V.about.108V are performed. Further, the processes subsequent to
step 108V, illustrated in FIG. 60, are explained with respect to
the case where stadless tires are equipped on two front wheels, and
radial tires are equipped on two rear wheels.
At steps 109V.about.117V, similar processes to those of the former
embodiments are performed.
However, at step 118V, when derived resonance frequency f.sub.K is
lower than, or equal to, reference resonance frequency f.sub.ST for
the stadless tire, or when f.sub.K is lower than, or equal to,
reference resonance frequency f.sub.RA for the radial tire, it is
determined that the air pressure of at least one tire is below the
allowable lower limit value. Thus, the process is advanced to step
119V, where an alarm is displayed to the driver on display portion
5. Otherwise, the process is returned to step 101V.
It should be noted that although the foregoing embodiment employs
f.sub.ST and f.sub.RA as reference resonance frequencies, it is
possible to use a difference (f.sub.STO -f.sub.KST or f.sub.RAO
-f.sub.KRA) between the resonance frequency f.sub.STO or f.sub.RAO
at the normal air pressure, or to use calculated resonance
frequencies f.sub.KST and f.sub.KRA, as reference resonance
frequencies.
In the twenty-third embodiment derived resonance frequency f.sub.K
is set equal to reference resonance frequency f.sub.KO when setting
switch 16 is turned on by the driver after a tire changing
operation. Therefore, the tire air pressure can be detected with
high precision irrespective of the type of new tires used.
The twenty-third embodiment will be explained with reference to the
flowchart of FIG. 61 as well as FIGS. 62 and 63. FIG. 62 is a
relationship between the resonance frequency and the tire air
pressure.
The processes of steps 201W-208W, are similar to those of the
twenty-second embodiment. However, at step 209W, the status of flag
F is compared to "1" where the flag F is reset to "0" only when the
ignition switch is turned OFF. Therefore, the process is advanced
to step 210W only when first tested; otherwise, the process is
advanced directly to step 211W. At step 210W, the state of setting
switch 16 is determined, where this switch is shown in FIG. 63.
If switch 16 is OFF in step 210W, the lowering difference between
resonance frequency f.sub.X, and reference resonance frequency
f.sub.KO is compared to reference difference .DELTA.f in step 211W,
where reference difference .DELTA.f is between the above-mentioned
frequency, f.sub.KO, and the resonance frequency, f.sub.L. Further,
this reference difference corresponds to the tire air pressure
lowering alarm limit as shown in FIG. 62.
For example, as set forth by step 211W, if the lowering difference
is less than, or equal to, the reference difference [(f.sub.KO
-f.sub.K).ltoreq..DELTA.f], the tire pressure is determined to be
within the allowable limit, and the process is redirected to step
201W. On the other hand, if the lowering difference is greater than
the reference difference [(f.sub.KO -f.sub.K)>.DELTA.f], the
tire pressure is determined to be below the allowable value and the
process is advanced to step 212W where an alarm is displayed for
the driver on display portion 5.
When switch 16 is in the ON state at step 210W, initial resonance
frequency f.sub.K is set equal to reference resonance frequency
f.sub.KO for each of the four wheels independently, at step 213W.
Then, at step 214W, flag F is set to "1", and the process returns
to step 201W.
Accordingly, after flag F has been set to "1" tire air pressure
detection is performed by comparing the difference between newly
set reference resonance frequency f.sub.KO and sequentially derived
resonance frequency f.sub.K, with difference .DELTA.f regardless of
the state of switch 16, where reference difference .DELTA.f is
measured between reference resonance frequency f.sub.KO and
resonance frequency f.sub.L.
It should be noted that while the reference resonance frequency
f.sub.KO can be set independently for each of the four wheels as
mentioned above, it is also possible to set reference resonance
frequency f.sub.KO equal to (1) an average value of resonance
frequencies f.sub.K derived with respect to each of the four
wheels, (2) an average value of two wheels excluding the maximum
and minimum values, or (3) the maximum or minimum value of
resonance frequencies f.sub.K, for each of the respective four
wheels.
The twenty-fourth embodiment is directed at performing a similar
process as described in the twenty-third embodiment, except that
the above-mentioned setting switch 16 is neglected. By setting
resonance frequency f.sub.K, which is derived immediately after
starting the tire pressure detecting process, equal to reference
resonance frequency f.sub.KO, any decrease in tire air pressure
which occurs during operation is detected irrespective of the tire
type.
The twenty-fourth embodiment will be explained with reference to
the flowchart of FIG. 64, wherein step 210W of the twenty-third
embodiment has been deleted as if switch 16 were in the ON
state.
Therefore, at step 209X, determination is made whether flag F is
set to "1" or not If it is not set to "1" the process is advanced
to step 213X so that resonance frequency f.sub.K is set equal to
reference resonance frequency f.sub.KO. On the other hand, when the
answer is positive, the process is advanced to step 211X for
further processing.
It should be noted that as in the twenty-third embodiment,
reference resonance frequency f.sub.KO can be set in the
above-mentioned manner of (1)-(3).
The twenty-fifth embodiment uses an effective rolling radius and
the unsprung mass resonance frequency to determine the tire type.
Namely, as shown in FIG. 65, effective rolling radius r.sub.S and
unsprung mass resonance frequency f.sub.S are linearly related
based on the type of the tire, where line x is normal radial tire,
line y is stadless tire, and line z is a low profile tire, each of
which corresponds to the previously explained types of tire. Using
a map of this type, the type of the tire can be determined with
r.sub.S and f.sub.S. For this reason, a tire changing judgment map,
similar to that of FIG. 65, is stored in ECU 4.
The process used to determine effective rolling radius r.sub.S and
unsprung mass resonance frequency f.sub.S as well as subsequent
determination of tire type, is illustrated by the flowcharts of
FIGS. 66 and 67.
At steps 101Y and 102Y, wheel speed v is derived on the basis of
the signal from the wheel speed sensor, and flag F is checked,
before the tire has been subjected to centrifugal force. It should
be noted that wheel speed v is calculated by waveshaping the output
signal of the wheel speed sensor, and by dividing the number of the
resultant pulses with a corresponding period.
If flag F is not set to "1", the process is advanced to step 103Y.
At step 103Y, vehicle speed V may be detected by means of a
doppler-type vehicle speed meter, or a rotational speed of a
transmission rotary shaft. Then, at step 104Y, tire load radius
r.sub.S is derived on the basis of both vehicle speed V and wheel
speed v.
At steps 105Y and 106Y, an FFT operation is performed with respect
to the wheel speed. The process is repeated until the number of
operation cycles of the frequency analysis, K, reaches a
predetermined number, K.sub.0, at which time it is advanced to step
107Y.
At steps 107Y and 108Y, the results of the frequency analysis are
averaged, and unsprung mass resonance frequency f.sub.S is
calculated based on this average.
At step 109Y, a map (FIG. 65) is used in conjunction with effective
rolling radius r.sub.S and unsprung mass resonance frequency
f.sub.S to determined the type tire. At step 110Y, alarming
reference values, f.sub.L and f.sub.H, are determined and stored
based on the tire type, where the values for each tire are shown on
the map of FIG. 68 as f.sub.La, f.sub.Lb, f.sub.Lc, f.sub.Ha,
f.sub.Hb, f.sub.Hc.
Only then is flag F is set to "1" at step 111Y. Thus, the
above-mentioned steps 103Y.about.110Y for determining tire type are
executed only immediately after vehicle ignition. In practice,
foregoing step 110Y is executed only when it is determined that all
four wheels or at least the two drive wheels have been changed in
step 109Y.
It should be noted that each process shown in FIG. 67 has been
described in the foregoing embodiments.
It should also be noted that the tire type discrimination of step
109Y may be performed with a regional map as illustrated in FIG.
69, instead of the linear map shown in FIG. 65. Accordingly,
discrimination between normal radial, stadless, and low-profile
tires is made based on the region mapped to by effective rolling
radius r.sub.S and unsprung mass resonance frequency f.sub.S.
Furthermore, determination of the tire type can be performed by
employing the matrix shown in TABLE 1. Namely, based on variation
in effective rolling radius r.sub.S and unsprung mass resonance
frequency f.sub.S, multiple matrices are formed based on the tire
types.
TABLE 1 ______________________________________ Tire Load Radius
(r.sub.s) Decreased Unchanged Increased
______________________________________ Unsprung Increased c c a
Mass (Low (Low (Normal Resonance Profile Profile Tire) Frequency
Tire) Tire) Unchanged c a b (Low (Normal (Stadless Profile Tire)
Tire) Tire) Decreased a b b (Normal (Stadless (Stadless Tire) Tire)
Tire) ______________________________________
For instance, if normal radial tires are used, a decrease in the
unsprung mass resonance frequency, which is caused by decreased
tire air pressure, will lead to a decrease in the effective rolling
radius. Conversely, an increase in tire air pressure results in an
increase in the effective rolling radius corresponding to an
increase in the unsprung mass resonance frequency. These
characteristics are designated by an "a" in the matrix shown in
TABLE 1.
Rubber used to construct stadless tires is softer and this results
in generally lower unsprung mass resonance frequencies. The
corresponding behavior is shown in blocks designated with a "b" in
TABLE 1.
On the other hand, the low profile tire generally has a high tire
spring constant. Therefore, the unsprung mass resonance frequency
is generally high, and the corresponding behavior is shown in
blocks designated with a "c" in TABLE 1.
It should be noted that when both r.sub.S and f.sub.S are either
increased or decreased, it is difficult to discriminate between
tire types. However, because changes in tire pressure are generally
different, the tire type can often be determined by aggregating the
results of discrimination of the other wheels.
For instance, when the unsprung mass resonance frequency and the
effective rolling radius are decreased simultaneously at two or
four of the wheels, it can be inferred that the tires have been
changed to stadless tires. Conversely, when both or all four tires
have risen, judgment can be made that the tires have been changed
to low profile tires.
Therefore, with the present embodiment, effects similar to those of
the foregoing embodiment can be achieved.
It should be noted that an optimal air pressure value for the
normal radial tire, or a value measured immediately before the
vehicle stops can be used for the above-mentioned reference values,
r.sub.0 and f.sub.0.
Next, the twenty-sixth embodiment will be discussed. Decreased tire
air pressure may be caused by natural leakage or puncture.
Generally, however, punctures cause these decreases. It is rare
that punctures are experienced by both left and right wheels
simultaneously. However, changing of the tire or wheel materials
results in a variation of the unsprung mass weight which affects
the vertical and longitudinal resonance frequency components in the
unsprung mass of the vehicle.
By deriving and comparing the resonance frequencies of the left and
right wheels with respect to each of the drive wheels and driven
wheels, judgment can be made that the tire air pressure has
decreased in the tire which has the lowest resonance frequency.
That is, only if there is definite difference between the resonance
frequencies. In the present embodiment, control is performed in
consideration of the above.
The foregoing process is illustrated in the flowcharts shown by
FIG. 70, where steps 101Z.about.108Z are similar to those described
in previous embodiments, and step 109Z is further described using
the flowchart of FIG. 71.
At step 201Z, resonance frequency f.sub.L, which is derived with
respect to the left side wheel of the front or rear, is compared
with the resonance frequency f.sub.R, which is derived with respect
to the right side wheel. Then, depending upon the results of
comparison between f.sub.L and f.sub.R, step 202Z or 203Z is
performed to set the higher resonance frequency as f.sub.MAX, and
the lower resonance frequency as f.sub.MIN.
When the unsprung mass weight is varied, the relationship between
the resonance frequency and the tire air pressure fluctuates, as
shown by the hatched region in FIG. 72. Therefore, any one of a
number of tire air pressures may result from one resonance
frequency. Thus, in step 204Z, a minimum value of the tire air
pressure, P.sub.MIN, which corresponds to f.sub.MIN, is derived
from a relationship between the resonance frequency (Hz) and the
tire air pressure (kg/cm.sup.2).
Then, the process is advanced to step 205Z, where minimum value
P.sub.MIN is compared with threshold level P.sub.TH to detect
abnormal decreases in tire air pressure.
If P.sub.MIN is less than P.sub.TH, the process jumps to step 209Z
to display an alarm indicative of the abnormal decreases in tire
air pressure on display portion 5. This process is a preventive
measure for the case where the tire air pressures of both of the
left and right wheels are lowered simultaneously.
Otherwise, if P.sub.MIN is greater than, or equal to P.sub.TH at
step 205Z, the process is advanced to step 206Z, where difference
.DELTA.f is derived from resonance frequencies f.sub.MAX and
f.sub.MIN of the left and right wheels. As set forth above, when
the unsprung mass weight is varied via tire variation, wheel
material or so forth, the characteristics between the resonance
frequency and the tire air pressure are also varied.
As shown in FIG. 73, .DELTA.f.sub.A corresponds to the difference
between normal resonance frequency f.sub.AN and abnormal resonance
frequency f.sub.AW, where f.sub.BN corresponds to normal tire air
pressure P.sub.N, and resonance frequency f.sub.BW corresponds to
the abnormally decreasing tire air pressure P.sub.W, as shown by
characteristic curve (B). Further, .DELTA.f.sub.B corresponds to
the difference between normal resonance frequency f.sub.BN and
abnormal resonance frequency f.sub.BW, where f.sub.BN corresponds
to normal tire air pressure P.sub.N and resonance frequency
f.sub.BW corresponds to the abnormally decreasing tire air pressure
P.sub.W, as shown by characteristic curve (B). Because difference
.DELTA.f.sub.A and difference .DELTA.f.sub.B may be different,
abnormal decreases in tire air pressure may be erroneously detected
by simply evaluating one or the other difference as .DELTA.f.
Thus, threshold level f.sub.TH is unconditionally determined for
judgment of abnormally low tire air pressure, where f.sub.TH is the
difference between the resonance frequencies.
If the variation of the unsprung mass coefficient factor in the
left and right wheels is caused only by the difference of the tire
air pressures at those wheels, then the unsprung mass coefficient
factors other than the tire air pressure, can absorb the influence
for the resonance frequency as follows. Characteristic charts, as
shown in FIG. 74, of the relationship between the maximum resonance
frequency and a difference of resonance frequencies for each tire
type can be detected. For instance, the normal tire air pressure
(e.g. 2.0 kg/cm.sup.2) and alarming tire air pressure (e.g. 1.4
kg/cm.sup.2) are derived with respect to various combinations of
tires and wheels. Then, using the characteristic lines indicated by
this data, the resonance frequency for each tire type is used to
determine the threshold corresponding to the other tire type.
It should be noted that the characteristic chart shown in FIG. 74
is stored in ECU 4. Accordingly, at step 207Z, threshold level
f.sub.TH is obtained from the map stored in ECU 4 with respect to
resonance frequency f.sub.MAX, and is regarded as normal tire air
pressure.
Then, at step 208Z, resonance frequency difference .DELTA.f is
compared with new threshold level f.sub.TH. If .DELTA.f is greater
than, or equal to f.sub.TH, the process is advanced to 209Z where
an alarm is displayed on display portion 5 to indicate an abnormal
decrease in the tire air pressure. On the other hand, if .DELTA.f
is less than f.sub.TH, the process simply returns to other
processing.
It should be noted that, depending upon the vehicle speed and
traveling condition, a specific tire air pressure may be determined
to be either dangerous or not dangerous. However, using a map of
many characteristic curves which take into account the vehicle
speed and travelling condition (FIG. 75), threshold level f.sub.TH
may be accurately derived.
It should also be noted that in the foregoing, the decreases in
tire air pressure can be determined by employing resonance
frequency f.sub.MAX instead of resonance frequency f.sub.MIN.
Selection of either f.sub.MIN or f.sub.MAX is made by taking the
degree of the tire air pressure decrease for the left and right
wheels into consideration. The relationship between the resonance
frequency and the tire air pressure shown in FIG. 72, is
preliminarily stored in the form of a map in ECU 4.
The foregoing embodiment can improve reliability by avoiding
erroneous detection of lowering abnormalities of tire air pressure.
Because the relationship between variation magnitude .DELTA.f and
the variation magnitude of the tire air pressure is affected by the
unsprung mass coefficient factor, threshold level f.sub.TH can be
corrected using resonance frequency f.sub.MAX which is regarded to
be normal tire resonance frequency.
On the other hand, by setting f.sub.MAX or f.sub.MIN as the
threshold value for judgment, natural leakage may be detected when
the tire air pressure of left and right wheels are lowered
simultaneously. Thus, with respect to fluctuation of the
characteristics between the resonance frequency and the tire air
pressure which depend upon the type of tire and wheel used, the set
threshold value for judgment may be adjusted by selecting from
among f.sub.MAX and f.sub.MIN.
The twenty-seventh embodiment is directed at improving detection
reliability by using a two-stage judgment to detect for decreases
in the tire air pressure. Specifically, after a variation rate of
resonance frequency f.sub.K is obtained for a unit period, the
variation rate is compared to a judgment value. Then, the number of
cycles in which the variation rate is less than this judgment value
are compared to predetermined value, Mo. Only after both
predetermined values have been exceeded will the alarm be set.
Thus, temporary fluctuations which erroneously result in detection
of decreased pressure are avoided.
The foregoing embodiment is illustrated by the flowchart of FIG.
76, where steps 101.alpha..about.108.alpha. are similar to those in
former embodiments.
However, at step 109.alpha., derived resonance frequency f.sub.K is
compared to predetermined air pressure lowering discrimination
value f.sub.L.
If f.sub.K is greater than f.sub.L, then the process is advanced to
step 114.alpha. where counter m is reset and the process is
returned to step 101.alpha..
However, if f.sub.K is less than, or equal to, f.sub.L, the process
is advanced to step 110.alpha. where resonance frequency variation
rate df.sub.K is compared with judgment value (.DELTA.f.sub.K
/.DELTA.t) to determine the degree of decrease in the tire air
pressure. It should be noted that .DELTA.f.sub.K is the difference
between the current calculated resonance frequency and previous
calculated resonance frequency, and that .DELTA.t is the elapsed
period therebetween.
When variation rate df.sub.K is greater than, or equal to,
foregoing judgment value (.DELTA.f.sub.K /.DELTA.t), the tire air
pressure is considered to be gradually decreasing. Therefore, the
process is advanced to step 111.alpha. to increment counter m.
Subsequently, at step 112.alpha., it is determined whether derived
variation rate of the derived resonance frequency has maintained a
level lower than judgment value for more than m.sub.o cycles.
If it has been lower for m.sub.o cycles, the answer at step
112.alpha. is positive, and the process is advanced to step
113.alpha. to display the alarm indicating low tire air pressure
for the relevant tire.
On the other hand, when it has not been lower for m.sub.o
continuous cycles, the answer at step 112.alpha. is negative, and
the process is returned to step 101.alpha..
Further, when the variation rate df.sub.K of the resonance
frequency is less than judgment value (.DELTA.f.sub.K /.DELTA.t) in
step 110.alpha., judgment is made that the tire air pressure is
abruptly lowered due to occurrence of abrupt decrease in tire air
pressure. Therefore, the process jumps to step 113.alpha. to permit
alarming display of the foregoing content.
It should be noted that, once initiated, the present embodiment
maintains alarming display until the vehicle stops. Then, upon
restarting the vehicle, if the initial resonance frequency f.sub.K
is higher than the air pressure lowering judging value, the tire
air pressure lowering detection state is released to terminate
alarming display. However, if the initial resonance frequency
f.sub.K is lower than, or equal to, the air pressure lowering
judgment value, the alarming display is maintained until the next
stop of the vehicle to repeat the foregoing steps.
The twenty-eighth embodiment is directed at determining a reference
value for judging abnormality in tire pressure based on
characteristics of the tire and wheel in the manner different from
the twenty-seventh embodiment.
As described in the twenty-third embodiment at FIG. 62, the tire
pressure, unsprung resonance frequency characteristic is
illustrated as shown in FIG. 77.
When the air pressure of the tire which is mounted on the lightest
wheel is lowered, the air pressure (kg/cm2) lower limit value and
upper limit value are taken as P.sub.L and P.sub.H, respectively.
Correspondingly, the reference/unsprung resonance frequency f.sub.L
for judging the decrease in air pressure in the radial tire is
indicated by F.sub.RAL, and the reference resonance frequency
f.sub.L for the stadless tire is indicated by f.sub.STL. It should
be noted that P.sub.L and P.sub.H may be set equal to the minimum
and maximum air pressures recited by the JIS, namely 1.4
gk/cm.sup.2 and 2.5 kg/cm.sup.2, respectively
In addition, because f.sub.KO is used to determine the type of tire
employed, it is desirable to set its value to a value centered
between f.sub.RAD and f.sub.STL, where f.sub.RAD and f.sub.STL are
the resonance frequency equivalents to upper limit pressure P.sub.H
of the stadless tire, and the lower limit pressure P.sub.L of the
radial tire, respectively.
The signal processing performed by ECU 4 in the twenty-eighth
embodiment will be described with reference to the flowchart of
FIG. 78.
It should be noted that the processes before step 170.beta. are
similar to those described by steps 100A to 160A of the first
embodiment.
However, in step 170.beta., longitudinal resonance frequency
f.sub.K and vertical resonance frequency f.sub.R of the unsprung
vehicle are determined based on the smoothed results of the FFT
operation (shown in FIG. 79). At step 180.beta., the status of flag
F is checked, where it is reset to "0" only after the ignition key
is turned off.
If flag F does not equal "1", at step 180.beta., the process
proceeds to step 190.beta. where resonance frequency f.sub.K is
compared to resonance frequency f.sub.KO in order to determine tire
type.
As shown in FIG. 77, the operated resonance frequency f.sub.K is
less than the resonance frequency f.sub.KO when the tire type is a
radial tire and the tire pressure is very low, and when the tire
type is considered to be stadless. Consequently, when resonance
frequency f.sub.K is less resonance frequency f.sub.KO, the tire
type can not be determined based on resonance frequency f.sub.KO
alone.
Wheel speeds (gains) v.sub.R and v.sub.K, which respectively
correspond to resonance frequencies f.sub.R and f.sub.K, are shown
in FIGS. 80(a) and 80(b). As shown, when the tire pressures of the
radial and stadless tires are decreased, the orientation of gains
v.sub.R and v.sub.K for the radial tire (see FIG. 80(a)) are
reversed, while the orientation of gains v.sub.R and v.sub.K for
the stadless tire (see FIG. 80(b)) do not change.
Consequently, using a ratio between the gains (v.sub.R /v.sub.K),
these tire types can be discriminated from each other.
For this reason, when resonance frequency f.sub.K is less than, or
equal to, f.sub.KO at step 190.beta., the process proceeds to the
step 200.beta., where gain ratio v.sub.R /v.sub.K is calculated and
compared with predetermined value .alpha.. Then, if gain ratio
v.sub.R /v.sub.K is greater than, or equal to, .alpha., it is
determined that the tire is stadless and the process is advanced to
step 220.beta..
However, if either resonance frequency f.sub.K is greater than
f.sub.KO at step 190.beta., or gain ratio v.sub.R /v.sub.K is less
than .alpha. at step 200.beta., it is determined that the tire is
radial, and the process is advanced to step 210.beta..
At steps 210.beta. and 230.beta., the tire weight (wheel weight) M
is determined based on the resonance frequency f.sub.R. A map is
then used to determine judgment resonance frequency f.sub.L
corresponding to tire pressure reduction warning pressure
P.sub.L.
If the tire spring constant and the unsprung weight are taken as k
and m, respectively, then the resonance frequency f.sub.R is
calculated by the following operational equation: ##EQU1##
Because unsprung weight m is a constant value which is determined
through vehicular data, when tire spring constant k is specified
through the tire air pressure, resonance frequency f.sub.R is based
on tire weight m. Thus, for example, three values are determined
for vertical resonance frequency f.sub.R, namely, f.sub.RA
corresponding to light tire weight, f.sub.RB corresponding to
intermediate tire weight, and f.sub.RC corresponding to light tire
weight (see FIG. 81). Then, the pressure-resonance frequency
characteristics are respectively calculated (see FIG. 82), and the
resultant map is recorded in ROM of ECU 4.
The values for judgment resonance frequencies f.sub.L1, f.sub.L2,
and f.sub.L3 corresponding to tire pressure reduction warning
pressure P.sub.L are then determined for resonance frequencies
f.sub.RA, f.sub.RB, and f.sub.RC, respectively.
Alternatively, if it is determined in steps 190-200.beta. that
stadless tires have been employed, the processes shown by steps
220.beta. and 240.beta. are performed, where these processes are
similar to those of steps 210.beta. and 230.beta..
When the determination of the tire type has been completed,
judgment resonance frequency f.sub.L corresponding to the tire
pressure reduction warning pressure P.sub.L is determined using the
map shown in FIG. 82, and flag F is set at "1" in step 250.beta..
Then, at step 260.beta., operated resonance frequency f.sub.R is
compared with resonance frequency f.sub.L.
When f.sub.R is less than, or equal to f.sub.L, the tire air
pressure is determined to be excessively low, and the process
proceeds to step 270.beta., where a warning is displayed to the
driver by display unit 5.
However, when f.sub.R is less than f.sub.L, the process is returned
to 100A.
It should be noted that because flag F is set equal to "1" after
this process has been completed, the processes from step 190.beta.
to 250.beta. are omitted after the first cycle.
In the above-described embodiment, the tire type is determined
based on a gain ratio between vertical resonance frequency f.sub.R
and longitudinal resonance frequency f.sub.k. It should be noted
that the determination of tire type may also be performed on the
basis of either a large or small relationship between resonance
frequencies f.sub.R and f.sub.k, or the deviation therebetween.
Further, the judgment for the above wheel weight may be performed
on the basis of the variation in maximum vertical resonance
frequency f.sub.R.
Consequently, because determination of the tire type is
automatically performed, and because the judgment value used to
compare the tire air pressure can be set according to the weight of
the wheel used to mount the tire, it is possible to achieve very
accurate detection of the tire pressure state.
When a vehicle is operated on a rough road, forces are applied to
both the tires and the suspension. Therefore, even if the tire
pressure is constant, the vertical or longitudinal resonance
frequencies under the unsprung state are decreased due to the
influence of non-linear characteristics of bushings and
vibration-proof rubber which are used in the suspension.
Additionally, when the vehicular speed is lowered, the signal level
(gain) used to determine resonance frequency is reduced. Thus, when
braking or slowing down, it may become impossible to accurately
detect the resonance point.
The twenty-ninth embodiment is directed at handling the
above-described problems through the processes illustrated in FIG.
83.
The processes before step 130.tau. are similar to steps 100A to
120A of the first embodiment, illustrated in FIG. 10. In subsequent
steps 130.tau. and 140.tau., an FFT operation and a data selection
process are executed. Specifically, in the data selection
processing, a selection lower limit determination value v.sub.L and
a selection upper limit determination value v.sub.H are determined
based on the waveform of the wheel speed. These values are compared
to peak value v.sub.P within predetermined frequency range f.sub.1
-f.sub.2.
If v.sub.P is less than, or equal to v.sub.L (FIG. 84(b)), or if
v.sub.P is less than, or equal to v.sub.H (FIG. 84(c)), then the
results of the FFT operation performed on corresponding portions
(A) and (B) are not used to determine resonance frequency
f.sub.K.
In the above step 140.tau., data selection is performed based on
the upper and lower limit values, v.sub.H and v.sub.L. However,
even in the data after selection (portion (C) in FIG. 84(a)), the
magnitude of the gain resulting from each FFT operation is
variable, as shown in FIG. 85. As a result, both the number of
averaging processes and the time required to calculate resonance
frequency, f.sub.K, are increased.
Consequently, in step 150.tau., the gain (magnitude) of the wheel
speed signal is adjusted by multiplying the result of each FFT
operational by a coefficient K.sub.1, K.sub.2, . . . K.sub.i.
Therefore, the peak values within the predetermined frequency range
(f.sub.1 to f.sub.2) are set equal to the predetermined value,
v.sub.PK, as shown in the resultant FFT operation waveform.
Finally, in subsequent step 160.tau., the FFT operational number N
is incremented, and the process is advanced to perform steps
similar to those after step 140A of the first embodiment.
In the thirtieth embodiment, a different data selection process and
gain adjustment process for the time-waveform of the wheel speed v
are performed. This process is shown by the flowchart of FIG.
86.
At step 100A, the wheel speed is calculated. Then, in the data
selection process of step 111.gamma., the selection of lower limit
judgment value .vertline.v.sub.L '.vertline. and the selection of
upper limit judgment value .vertline.v.sub.H '.vertline. are made
as shown in FIG. 87. Then, only the time-waveform of wheel speed v
having a magnitude within the range of (-v.sub.H ' TO -v.sub.L '),
or (-v.sub.L ' TO -v.sub.H ') is considered.
Correspondingly, the gain adjustment in step 112.gamma., is
performed by multiplying wheel speed v within specified time
.DELTA.t' by coefficient k.sub.1 '. Then, after the data selection
process, the peak value within specified time .DELTA.t', which is
designated by v.sub.p ', is set equal to a predetermined value, as
shown in FIG. 88. The processes after step 112.gamma. are then
performed, as described with respect to the first embodiment, to
detect decreases in the tire pressure as described with respect to
the first embodiment.
In the above-described twenty-ninth and thirtieth embodiments, by
executing a data selection process, resonance frequency f.sub.K for
detecting the tire pressure can be determined without being
lowered.
Further, since the gain adjustment process is performed, even when
the frequency characteristic is changed, the magnitude of the gain
is not variable, as result of each FFT operation. It is therefore
impossible to reduce the number of averaging processes required for
calculating the average of the FFT operational results.
Consequently, in the twenty-ninth and thirtieth embodiments, it is
difficult to rapidly detect lowering of the tire pressure.
It should also be noted that in each above-mentioned embodiments,
the value of the tire air pressure, as well as the abnormal alarm
of the tire air pressure may be displayed directly.
Although the embodiments have been disclosed in detail, the present
invention should not be limited to these embodiments. For instance,
in FIG. 4, it is possible to detect the tire air pressure on the
basis of variation of gain at a specific frequency or variation of
the frequency at a specific gain.
As set forth above, according to the present invention, the
predetermined frequency component in the tire vibration frequency
component varies according to variation of the spring constant of
the tire. Therefore, the air pressure condition of the tire is
detected based on the variation of the frequency component of a
tire. Consequently, the vehicular occupant can monitor the tire air
pressure while traveling in the vehicle. In addition, by employing
a device which adjusts the tire air pressure during travel, driving
performance can be significantly enhanced.
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